Add a Brackets-blue highlight to the selected working file

It looks like this: 
![highlight](http://f.cl.ly/items/2w1S2p1V0c3N22253K28/Screen%20Shot%202013-08-14%20at%202.57.37%20PM.png "highlight")

The idea is from @larz0, who made a similar tweak to Edge Code that I now can't live without. (@larz0 also approved this particular blue highlight color.)

CC @njx :cake: