take context info from the command line

[srl295]

Thanks for billwarrior, I'm about to bill my first client using it.

It's very convenient to run timew report billwarrior, but I can see it being a challenge to produce reports for more than one client which might need different configuration files.

Options:

1. don't (only) run as an extension, but run outside of the extension system so that more arguments could be made:

$ timew export | billwarrior config=client1 … 

2. Probably a better option, would be to use some configuration option, see https://github.com/GothenburgBitFactory/timewarrior/issues/230#issuecomment-517126353

maybe:

$ timew myreport rc.billwarrior.config=client1  | tee billwarrior_items.tex …

and then have billwarrior.ini have multiple subsections for client1, client2 etc