Feature: Web Panic Reporting via hooks

This PR is for issue of "email after registry webapp panic" #41, improving my
previous design (closed).
It use self setting up hooks, to catch panic in web application.
And, send email in hooks handle directly, to no use new http server and
handler.

Signed-off-by: xiekeyang <keyangxie@126.com>

cmd/registry/config.yml

@@ -4,6 +4,20 @@ log:
   fields:
     service: registry
     environment: development
+  hooks:
+    - type: mail
+      disabled: true
+      levels:
+        - panic
+      options:
+        smtp:
+          addr: mail.example.com:25
+          username: mailuser
+          password: password
+          insecure: true
+        from: sender@example.com
+        to:
+          - errors@example.com
 storage:
     cache:
         blobdescriptor: redis
@@ -41,5 +55,5 @@ notifications:
           timeout: 1s
           threshold: 10
           backoff: 1s
-          disabled: true
-   
+          disabled: true 
+