Invoke hook when server starts (#3024)

* Invoke hook when server starts

Invokes the `th-server-command-start` hook when the server is started, with the server object as the parameter. This allows adding a WebSocket listener to the server.

* Return the HTTP server from the listen function

Returns the node HTTP server created in the listen function to allow extension

* Add node HTTP server to server-command-start hook

* Change hook to post start in case we add a pre-start hook

* Create Hook__th-server-command-post-start.tid

editions/dev/tiddlers/new/Hook__th-server-command-post-start.tid

@@ -0,0 +1,18 @@
+created: 20180409142128584
+modified: 20180409142128584
+tags: HookMechanism
+title: Hook: th-server-command-post-start
+type: text/vnd.tiddlywiki
+
+This hook allows plugins to extend the TiddlyWiki server command after it initializes. The two 
+most obvious use cases are adding routes (such as an attachments folder for external files) 
+to the SimpleServer instance and adding a websockets handler to the HTTP server. 
+
+Hook function parameters: 
+
+* SimpleServer instance
+** Defined in core/modules/commands/server.js
+* NodeJS HTTP Server instance
+** See the NodeJS docs at [ext[https://nodejs.org/docs/latest-v8.x/api/http.html#http_class_http_server]]
+
+Return value is ignored.