RhodeCode CE/EE

add renderer to per-repo-settings. It will allow to define what renderer each repository uses. Probably need some changes how we fetch the global renderers.
per repo renderer will allow to generate per-repo order of readmes.

added new option about readme generation to into global visual and per repo: visual, consisting of:
autodetect (based on renderer try to auto-detect like currently)
none (don't load any readmes),
List in order of picked renderer either global, or local