[Markdown] [BBCode] Implement formatting buttons

_From @tobscure on August 27, 2015 13:54_

What needs to be done:
- Work out which formatting buttons to add, and how they should be implemented among multiple formatting extensions (Markdown/BBCode). Should each formatting extension implement its own formatting buttons with the same keys, and then the extension that's enabled last would "win" but buttons that it doesn't overwrite would still display?
- I envision they should be added to the TextEditor controls area (to the right of the submit/preview buttons); we should make sure this looks good and there's enough room for it. Could also consider putting them in a little popup that appears when you click on a button in that toolbar (like Google Inbox).

_Copied from original issue: flarum/markdown#1_

Start by locating the Markdown and BBCode formatting-extension entry points and the TextEditor controls area mentioned in the issue. Review how multiple extensions could provide shared formatting-button keys and decide whether controls belong beside submit/preview or in a popup. No file or test is named; done requires an agreed interaction and implementation that works across the enabled formatting extensions.

Written by the indexing model from the issue text.