add support for interactive selection with fzf (#74)

Present a fuzzy search choice in "kubectx" and "kubens" commands without
arguments.

![demo2](https://user-images.githubusercontent.com/159209/44478683-40f16d00-a5f3-11e8-99e2-f32f2a3539c1.gif)

Fixes #71.

README.md

@@ -127,7 +127,17 @@ sudo apt install kubectx
 
 -----
 
-### Customizing current context colors
+### Interactive mode
+
+If you want `kubectx` and `kubens` commands to present you an interactive menu
+with fuzzy searching, you just need to [install
+`fzf`](https://github.com/junegunn/fzf) in your PATH.
+
+![kubectx interactive search with fzf](img/kubectx-interactive.gif)
+
+-----
+
+### Customizing colors
 
 If you like to customize the colors indicating the current namespace or context, set the environment variables `KUBECTX_CURRENT_FGCOLOR` and `KUBECTX_CURRENT_BGCOLOR` (refer color codes [here](https://linux.101hacks.com/ps1-examples/prompt-color-using-tput/)):
 

kubectx

@@ -89,6 +89,17 @@ switch_context() {
   kubectl config use-context "${1}"
 }
 
+choose_context_interactive() {
+  local choice
+  choice="$(FZF_DEFAULT_COMMAND='kubectx' fzf --ansi || true)"
+  if [[ -z "${choice}" ]]; then
+    echo 2>&1 "error: you did not choose any of the options"
+    exit 1
+  else
+    set_context "${choice}"
+  fi
+}
+
 set_context() {
   local prev
   prev="$(current_context)"
@@ -170,7 +181,11 @@ delete_context() {
 
 main() {
   if [[ "$#" -eq 0 ]]; then
+    if [[ -t 1 && "$(type fzf &>/dev/null; echo $?)" -eq 0 ]]; then
+      choose_context_interactive
+    else
       list_contexts
+    fi
   elif [[ "${1}" == "-d" ]]; then
     if [[ "$#" -lt 2 ]]; then
       echo "error: missing context NAME" >&2

kubens

@@ -85,6 +85,25 @@ switch_namespace() {
   echo "Active namespace is \"${2}\".">&2
 }
 
+choose_namespace_interactive() {
+  # directly calling kubens via fzf might fail with a cryptic error like
+  # "$FZF_DEFAULT_COMMAND failed", so try to see if we can list namespaces
+  # locally first
+  if [[ -z "$(list_namespaces)" ]]; then
+    echo >&2 "error: could not list namespaces (is the cluster accessible?)"
+    exit 1
+  fi
+
+  local choice
+  choice="$(FZF_DEFAULT_COMMAND='kubens' fzf --ansi || true)"
+  if [[ -z "${choice}" ]]; then
+    echo 2>&1 "error: you did not choose any of the options"
+    exit 1
+  else
+    set_namespace "${choice}"
+  fi
+}
+
 set_namespace() {
   local ctx prev
   ctx="$(current_context)"
@@ -137,7 +156,11 @@ swap_namespace() {
 
 main() {
   if [[ "$#" -eq 0 ]]; then
+    if [[ -t 1 && "$(type fzf &>/dev/null; echo $?)" -eq 0 ]]; then
+      choose_namespace_interactive
+    else
       list_namespaces
+    fi
   elif [[ "$#" -eq 1 ]]; then
     if [[ "${1}" == '-h' || "${1}" == '--help' ]]; then
       usage