Introducing the --base switch to modify the base URL.

The --base option (defaulting to "/") essentially configures the
base_url in http.c which was introduced previously.  Now darkstat works
without too much magic behind a reverse proxy.

My mod_proxy config changed from
        ProxyPass /darkstat http://localhost:666
        <Location /darkstat/>
                ProxyPassReverse /
                SetOutputFilter proxy-html
                ProxyHTMLURLMap / /darkstat/
                RequestHeader unset Accept-Encoding
        </Location>
        Redirect permanent /graphs.xml /darkstat/graphs.xml
to the simpler
        ProxyPass /darkstat/ http://localhost:666/darkstat/
        ProxyPassReverse /darkstat/ http://localhost:666/darkstat/
(the ProxyPassReverse isn't even needed since darkstat doesn't generate
any Location headers).