Added support for custom ancestors: g:ctrlp_custom_ancestors.

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`:

`let g:ctrlp_custom_ancestors = ['pom.xml']`