Add command and keybinding for toggling VCS ignored files

Fixes #170

This binds a command to toggle VCS ignored files to the `i` key, as suggested in #170:

> Yeah, I like this idea, maybe just the i keybinding when the tree view is focused? i for ignore.

This works as expected, but also collapses all open folders (because the whole tree view is updated):

![](https://cloud.githubusercontent.com/assets/38924/3376726/02cdc574-fbd8-11e3-86eb-db2e1587fb3c.gif)

(In the GIF, I expand two folders, then press `i` a few times.)

I'm fine with this behavior, but still wanted to point it out in case this is a :scream:. This is also how the tree view behaves when you flip the option in Atom > Preferences.

cc @kevinsawicki @kuatsure