Show the byfname toggle in status line of CtrlP

I added the visualization of the `byfname` toggle (ctrl-d by default) to the status line in CtrlP, with the same presentation as the existing `regex` toggle.

![1416490763](https://cloud.githubusercontent.com/assets/1671603/5125337/fc8f1e88-70fd-11e4-9496-59eb54254c66.png)