Improve CLI help documentation

[peternewman]

I suspect this might be partly down to indexzero/nconf#275 but it would be nice if there was some help other than --help/--version. I assumed that there weren't options initially.

If nothing else it might be good to just override the usage so it mentions e.g. --casper.config foo/casparcg.config or something (which feels like it must be one of the most frequent options I'd imagine), and maybe a note to say other config options are available?