Add 'editor:save-debug-snapshot' command. Closes #191.

This saves the state of the rendered lines, the display buffer, the
tokenized buffer, and the buffer to a file. If a problem arises with
rendering, hopefully we can use it to diagnose in which layer things
went awry.

src/app/edit-session.coffee

@@ -820,5 +820,11 @@ class EditSession
       @displayBuffer.tokenizedBuffer.resetScreenLines()
     grammarChanged
 
+  getDebugSnapshot: ->
+    [
+      @displayBuffer.getDebugSnapshot()
+      @displayBuffer.tokenizedBuffer.getDebugSnapshot()
+    ].join('\n\n')
+
 _.extend(EditSession.prototype, EventEmitter)
 _.extend(EditSession.prototype, Subscriber)