Add option to allow locking a plugin to a specific revision

[annagrram]

An attempt to, at least, start the 'lock to revision' feature.
I've run some tests, but I guess they were far from perfect.
Looking forward to bugs and ideas 😄

As a side effect, also takes care to mark and not to update "detached head" repositories.

[opsroller]

I like this, makes it more like a Gemfile. Maybe one day we can have a Vimfile for directory specific settings, and plugins.

[thomasba]

@btreeinfinity Well there is a plugin called localrc ;-)
Sadly you can’t use it to load other plugins :-(

[opsroller]

even if we had a global file @ ~/.vundler that had the actual plugins you want to use, with all the changes/features I see people asking for like the ability to set branches/tags, etc.. having a file just for Vundle would be a good solution.