Make prefix in completion menu of separate color

When using zsh completion menu, the already-typed prefix is made faint (decreased intensity) and striked-through, and the next character is made bold (increased intensity).

This makes it easy to quickly see which is the next character that distinguishes available completion options.

Reddit thread: http://redd.it/msps0
Solution thanks to: @nicoulaj

To preview, set this and then complete on something in `/usr/bin/` (or somewhere with lots of files with common prefixes).