Default buffer encoding to core.fileEncoding config

Adds `core.fileEncoding` which will be used as the editor/buffer's default encoding when reading/writing.

I went with `core` instead of the `editor` for the prefix since this default should be respected for any packages reading/writing files, regardless of whether they are using the editor/buffer API or not.

![screen shot 2014-11-10 at 5 59 01 pm](https://cloud.githubusercontent.com/assets/671378/4987134/655ab9dc-6946-11e4-93a4-dc3252134c3e.png)

Closes atom/encoding-selector#3