Add something else other than files, buffers and MRU files for CtrlP to search for and perform any action on.
There are 2 points of entry:
Provide ctrlp with a simple list of strings to search in.
Specify an action to perform on the selected string.
To see how it works, get the sample.vim from this branch and place it (along with the directories) somewhere in your runtimepath. Then put this into your vimrc:
let g:ctrlp_extensions = ['sample']
A 4th search type will show up when you open CtrlP.
Check out sample.vim for more details