Add support for automatic and custom clone urls

This adds support for two new parameters to cgitrc: clone-prefix and
repo.clone-url.

If clone-prefix is specified, all repos will get a clone url printed in the
sidebar; the url is generated by clone-prefix + repo.url.

Additionally, each repo can specify repo.clone-url which will override any
such auto-generated url.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>