Add argument parsing + switch for uncached operation

This adds support for the following options to cgit:
  --root=<path>
  --cache=<path>
  --nocache
  --query=<querystring>
  --repo=<reponame>
  --page=<pagename>
  --head=<branchname>
  --sha1=<sha1>
  --ofs=<number>

On startup, /etc/cgitrc is parsed, followed by argument parsing and
finally querystring parsing.

If --nocache is specified (or set in /etc/gitrc), caching is disabled and
cgit instead generates pages to stdout.

The combined effect of these two changes makes testing/debugging a lot
less painfull.

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