Add a new command to pull up git status and switch to a file from the list of changes.

@bbatsov I find myself wanting to just see a list of modified files and switch to one of them as soon as I open up a project, so I added this command.

![Screenshot](https://www.evernote.com/shard/s2/sh/cfa1d2e3-5cde-42d5-b83e-25e65918b4c1/c15ed7353276d9135412a833f8780f50/res/80f260b4-90bc-4a96-b6ba-4481232168eb/skitch.png)

There's a little extra complication because I wanted to see whether the file was modified, added, etc (the ` M` or `??`). But it gets the job done (`projectile-git-changed` gets the list of files that we want, essentially filtering out deleted files, and `projectile-git-status` gets the status strings for each file).

But I could use your advice on what's going on with `completing-read` here. It seems that when you press enter without typing anything, it passes in an empty string instead of the first item in the list.
There should be a vc-independent command calling the appropriate vc commands based on the project type. I don't want us to hardcode something that's git specific. You can raise an error for other VCS if you don't want to implement support for them.