Fix highlighting for custom ctrlp_line_prefix.
Fix highlighting for custom ctrlp_line_prefix.

Before this commit, if the user defined `g:ctrlp_line_prefix`, then such
prefix would be shown in every line (while the default prefix is shown
only in the active line). This commit makes a custom prefix behave like
the default one.

escape() is needed to escape magic characters.

tschundler/dotfiles,tschundler/dotfiles,tschundler/dotfiles