Plugin: remote plugins

[samnung]

It would be great if you could create common plugins and use theme in every book and not have to rely on git submodules or subtrees.

• allow to fetch plugins from web (ruby file, zip folder containing multiple files, GitHub gist)
• allow to fetch plugins from public and private Git repo (select branch, tag or commit)

"Steal" code from CocoaPods to deal with this stuff, they have already solved this (or maybe use it as library?).