Allow multiple builders in one run of the plugin

[tomdz]

Apparently, the site plugin doesn't seem to support run the same reporting plugin twice, but with different configurations.

[rgoers]

I use a profile to generate pdf's. So when I do -P pdf it uses that configuration and ignores the default. It would be great if I could specify a different conf.py somehow though.

[tomdz]

I created #11 for specifying a different conf.py since Sphinx supports that out of the box (-c option).