ENH: Allow recursive file listing.

This can be toggled via <F7>. Also switch the mapping for toggling the original files from T (which is a useful motion also when renaming) to <F6>.
Use default highlight groups, retire config vars.

Vim has a built-in way to define default highlightings while allowing customization: hi def link. Replace the various g:RenamerHighlight... configuration variables, and also the cumbersome and only tangential implementation of modifying an existing highlight group by adding "bold".