Allow passing arbitrary extra args to rrdgraph in the master configuration

[ToxicFrog]

Is your feature request related to a problem? Please describe.

Individual plugins can set graph_args to control how rrdgraph is invoked, but there is no way to do this at the server level to (say) globally change the graph colours or font used. The rest of the web UI can be reskinned with CSS, but the graphs remain out of reach.

Describe the solution you'd like

A graph_args setting at the munin-master level, which contains extra arguments to be inserted into the rrdgraph argv after the rrd_header but before the per-plugin graph_args.

Describe alternatives you've considered

• CSS filters on <img>. These work, kind of, but you can't do adequate hue rotation in CSS and it's impossible to adjust individual colours, font, border, etc.
• update the images with mogrify after each munin-cron run. I did this for a while; you still can't adjust individual graph elements but if you're doing colour manipulations it does at least work better than CSS. It's slow, though, and only works for graph_strategy cron.
• wrap rrdgraph with a custom script that injects the desired arguments. This doesn't work because munin uses the rrdgraph library interface rather than invoking an external command.

Additional context

This was previously discussed on IRC, I'm just promoting it to an FR so that it doesn't get lost.