Rename `comment` field to `content`

For consistency. You will see why later.
Pass `site` to `find_for` instead of `source`

This will become clear in a minute.
Format the comment (converter determined by filename extension)

It's so pretty! Oh so pretty! Oh so pretty, and witty, and gaaay!
And I pity, any comment not as pretty as theeee!
Make sure the filename extension in the config isn't missing a period

_Never_ trust the user to not botch things up.
Merge branch 'dev'