Fix #11, lighter region face.

This makes the comments within a selection clearly visible. Per screenshots in the bug report, comments are not visible within selections.

The approach in many themes is similar, by using a colour a bit darker than the light highlight or lighter than the default background.

![screen shot 2014-10-13 at 3 22 43 pm](https://cloud.githubusercontent.com/assets/5206067/4618877/a39f5df4-530e-11e4-8ddd-465d1863351d.png)

On the `company-tooltip-selection` line, the full line is not selected and there's a small contrast between the region and the line highlight colour. It seems pretty common to do that for many themes.