Extend IEx.DocsFormatter to support full list item notation

[christhekeele]

@josevalim mentioned that only * is currently supported. Adding - and + would allow it to comply with the Daring Fireball markdown 'spec'.

I might take a stab at this tonight.

[josevalim]

Sorry, I just did it. :) Your issue came just in time for me to mention it in the commit title.

[christhekeele]

Too quick for me. Shouldn't + also be supported?

[josevalim]

Added support for + too. Thanks for sanity checking!