Octicon-based favicons for tabs

While playing with https://github.com/atom/settings-view/issues/80 today, I though it might be :cool: to have a simple way to add octicons to tabs (as favicons):

![screen shot 2014-04-26 at 9 53 27 pm](https://cloud.githubusercontent.com/assets/38924/2809673/8167a52e-cd7c-11e3-94b1-36ff9493e0b4.png)

For example, those would be helpful when looking for "special" tabs, like settings and search results. The [SettingsView](https://github.com/atom/settings-view/blob/master/lib/settings-view.coffee) and [ResultsPaneView](https://github.com/atom/find-and-replace/blob/master/lib/project/results-pane.coffee) would then just be extended with a `getIcon` method which returns the name of the octicon to be used (e.g. `tools` and `search`).

Thoughts? :doughnut: If this makes sense, I'd look into adding some tests. :cactus:  

cc @kevinsawicki @Caged (semi-related to https://github.com/atom/tabs/issues/41)