Allow disabled packages to be installed, take 2

This is a second attempt at https://github.com/atom/settings-view/pull/74, which fixes #71.

Currently, disabled packages can't be installed via Settings. This PR introduces two changes:
* disabled packages can now be installed after searching for them (same as #74)
* disabled packages are automatically re-enabled after being installed

:movie_camera: :clapper: 

![testy](https://cloud.githubusercontent.com/assets/38924/3142233/94248c98-e9b2-11e3-88d0-16111d926c56.gif)

cc @probablycorey