Markdown Statuses

[tsmethurst]

This PR adds functionality for parsing statuses in markdown format, using https://github.com/russross/blackfriday.

To submit a status in markdown format, a new field must be set when submitting a status through the api:

	// Format in which to parse the submitted status.
	// Can be either plain or markdown. Empty will default to plain.
	Format StatusFormat `form:"format" json:"format" xml:"format"`

In other words, when doing a POST request to /api/v1/statuses you can now include something like -F format=markdown or -F format=plain to specify what kind of status you're submitting and how you'd like it to be parsed.

To avoid breaking anything, the default behavior when the field is not specified is to assume plaintext.

Unfortunately, no clients yet support this functionality (as far as I'm aware). But it should be useful when submitting statuses through the API, and hopefully this functionality can be added to some clients in the future :)

Note: to avoid completely breaking federation with spammy, weird-looking posts, table-formatting is not supported, because tests showed that it looked terrible in other apps (Mastodon).

Closes #110