Add support for github style checklists

Evernote has checkboxes to signify todo items but these are stripped out
when showing or editing the note.  Add support for todo items by using
'[ ]' for incomplete and '[x]' for complete todo items.  This is
consistent with github style markdown.
Merge branch 'master' of https://github.com/MattHulse/geeknote

Conflicts:
	geeknote/editor.py