Adding source dialog to the plugin build

I would like HTML edit mode for articles. I'm currently editing by inspecting HTML and changing the content editable element, but that's getting fairly annoying.

Since we use the inline editor mode, the way to do this is actually the "sourcedialog" plugin, not "sourcearea", so I've added that plugin and re-built

Test Plan:
Add 'Sourcedialog' to toolbar and extraPlugins='sourcedialog' to config, see Source show up on toolbar in article editor.
Merge pull request #2 from Khan/pf-add-sourcedialog

Adding source dialog to the plugin build