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>

docs/configuration.md

@@ -25,6 +25,20 @@ log:
 	fields:
 		service: registry
 		environment: staging
+	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
 loglevel: debug # deprecated: use "log"
 storage:
 	filesystem:
@@ -229,6 +243,28 @@ log:
     </td>
 </table>
 
+## hooks
+
+
+```yaml 
+hooks:
+  - type: mail
+    levels:
+      - panic
+    options:
+      smtp:
+        addr: smtp.sendhost.com:25
+        username: sendername
+        password: password
+        insecure: true
+      from: name@sendhost.com
+      to:
+        - name@receivehost.com
+```
+
+The `hooks` subsection configures the logging hooks' behavior. This subsection
+includes a sequence handler which you can use for sending mail, for example.
+Refer to `loglevel` to configure the level of messages printed.
 
 ## loglevel