Added support for custom ancestors: g:ctrlp_custom_ancestors.

From https://github.com/kien/ctrlp.vim/pull/599:

Working path mode 'r' looks for the following files in ancestors: `.git .hg .svn .bzr _darcs`. Some projects, however, don't have any of these.

This PR implements support for declaring custom ancestor filenames, e.g. `pom.xml`:

``` vim
let g:ctrlp_custom_ancestors = ['pom.xml']
```