Incorporate the rich text editor

[AngeloStavrow]

From one of our TestFlight users:

You should incorporate the rich text editor from the website into the app. Just a suggestion! :-)

Technically we could build a rich-text editor of sorts for the app. This will require some design work to determine the best approach (a keyboard toolbar, probably?), and combined with #91 ("preview a post") could be a compelling option for users that aren't especially fond of reading Markdown.