Support customized RRD URLs

Use case: I would like to bypass PHP for serving the RRD files and allow
the webserver (nginx) to compress it to reduce load on the Raspberry Pi.
I could go through all kinds of URL rewriting, but it is much easier to
set `$CONFIG['rrd_url'] = 'rrd/{file}';` instead and add a corresponding
location + alias directive to the nginx configuration.

This depends on https://github.com/manuelluis/jsrrdgraph/pull/17 to avoid breaking on `=`.

Both forms are tested and are correctly processed (provided that you include that jsrrdgraph patch!).