[Rivet] rivet-mkhtml config file

[Giancarlo Panizzo]

Dear rivet authors, 

I would like to let rivet-mkhtml pass a config file to make-plots through the "-c" command line option. 

While the option is correctly passed to cmp-histos, this seems not to be the case for the last make-plots command, launched internally by rivet-mkhtml. This in turn seems to basically cause rivet-mkhtml to ignore such config file parameter.

What I would propose is to replicate somehow the lines

## Pass rivet-mkhtml -c args to rivet-cmphistos
261	for configfile in opts.CONFIGFILES:
262	    configfile = os.path.abspath(os.path.expanduser(configfile))
263	    if os.access(configfile, os.R_OK):
264	        ch_cmd += ["-c", configfile]


also after line 452, of course replacing ch_cmd -> mp_cmd.

I hope this will help (at least it fixes the issue for me), and of course thanks for your efforts in developing such a successfull project!

Best

Giancarlo Panizzo