Adds "list-tags" command to list tags with no known tag required. Fixes #276

Example: skopeo list-tags docker://docker.io/library/centos
Returns response:
{
  Repository": "docker.io/library/centos",
  "Tags": [
    "6",
    "7",
    ...
  ]
}

Signed-off-by: Zach Hill <zach@anchore.com>

docs/skopeo-list-tags.1.md

@@ -0,0 +1,102 @@
+% skopeo-list-tags(1)
+
+## NAME
+skopeo\-list\-tags - Return a list of tags the transport-specific image repository
+
+## SYNOPSIS
+**skopeo list-tags** _repository-name_
+
+Return a list of tags from _repository-name_ in a registry.
+
+  _repository-name_ name of repository to retrieve tag listing from
+
+  **--authfile** _path_
+
+  Path of the authentication file. Default is ${XDG\_RUNTIME\_DIR}/containers/auth.json, which is set using `podman login`.
+  If the authorization state is not found there, $HOME/.docker/config.json is checked, which is set using `docker login`.
+
+  **--creds** _username[:password]_ for accessing the registry
+
+  **--cert-dir** _path_ Use certificates at _path_ (\*.crt, \*.cert, \*.key) to connect to the registry
+
+  **--tls-verify** _bool-value_ Require HTTPS and verify certificates when talking to container registries (defaults to true)
+
+  **--no-creds** _bool-value_ Access the registry anonymously.
+
+## REPOSITORY NAMES
+
+Repository names are transport-specific references as each transport may have its own concept of a "repository" and "tags". Currently, only the Docker transport is supported.
+
+This commands refers to repositories using a _transport_`:`_details_ format. The following formats are supported:
+
+  **docker://**_docker-repository-reference_
+  A repository in a registry implementing the "Docker Registry HTTP API V2". By default, uses the authorization state in either `$XDG_RUNTIME_DIR/containers/auth.json`, which is set using `(podman login)`. If the authorization state is not found there, `$HOME/.docker/config.json` is checked, which is set using `(docker login)`.
+  A _docker-repository-reference_ is of the form: **registryhost:port/repositoryname** which is similar to an _image-reference_ but with no tag or digest allowed as the last component (e.g no `:latest` or `@sha256:xyz`)
+      
+      Examples of valid docker-repository-references:
+        "docker.io/myuser/myrepo"
+        "docker.io/nginx"
+        "docker.io/library/fedora"
+        "localhost:5000/myrepository"
+        
+      Examples of invalid references:
+        "docker.io/nginx:latest"
+        "docker.io/myuser/myimage:v1.0"
+        "docker.io/myuser/myimage@sha256:f48c4cc192f4c3c6a069cb5cca6d0a9e34d6076ba7c214fd0cc3ca60e0af76bb"
+       
+
+## EXAMPLES
+
+### Docker Transport
+To get the list of tags in the "fedora" repository from the docker.io registry (the repository name expands to "library/fedora" per docker transport canonical form):
+```sh
+$ skopeo list-tags docker://docker.io/fedora
+{
+    "Repository": "docker.io/library/fedora",
+    "Tags": [
+        "20",
+        "21",
+        "22",
+        "23",
+        "24",
+        "25",
+        "26-modular",
+        "26",
+        "27",
+        "28",
+        "29",
+        "30",
+        "31",
+        "32",
+        "branched",
+        "heisenbug",
+        "latest",
+        "modular",
+        "rawhide"
+    ]
+}
+
+```
+
+To list the tags in a local host docker/distribution registry on port 5000, in this case for the "fedora" repository:
+
+```sh
+$ skopeo list-tags docker://localhost:5000/fedora
+{
+    "Repository": "localhost:5000/myapp",
+    "Tags": [
+        "latest",
+        "v1.1.1",
+        "v1.2.0"
+    ]
+}
+
+```
+
+# SEE ALSO
+skopeo(1), podman-login(1), docker-login(1)
+
+## AUTHORS
+
+Zach Hill <zach@anchore.com>
+

docs/skopeo.1.md

@@ -75,6 +75,7 @@ Most commands refer to container images, using a _transport_`:`_details_ format.
 | [skopeo-standalone-sign(1)](skopeo-standalone-sign.1.md)    | Sign an image.                                               |
 | [skopeo-standalone-verify(1)](skopeo-standalone-verify.1.md)| Verify an image.                                             |
 | [skopeo-sync(1)](skopeo-sync.1.md)| Copy images from one or more repositories to a user specified destination.             |
+| [skopeo-list-tags(1)](skopeo-list-tags.1.md)  | List the tags for the given transport/repository.                           |
 
 ## FILES
   **/etc/containers/policy.json**