Speedup scoped config

Turns out `atom.config.get()` is called a ton when rendering the editor. This pull fixed some of it: https://github.com/atom/scoped-property-store/pull/4

But it was still noticeably slower when opening a tab. Here are some profiles:

Before:
![screen shot 2014-10-10 at 12 33 45 pm](https://cloud.githubusercontent.com/assets/69169/4619076/f477ef64-5310-11e4-87bd-5df284015c60.png)

After. No more config junk at the top of the list
![screen shot 2014-10-13 at 12 35 03 pm](https://cloud.githubusercontent.com/assets/69169/4619086/013be066-5311-11e4-9acf-8c359f092450.png)

It also feels much better.