Have a more local --local install

[perrette]

Right now the papers install --local writes .papersconfig locally, but keep the same global defaults for filesdir and bibtex
While this may be OK and can be overwritten manually, the use case for --local is to have parallel, independent, medium-size projects. To me, this is one of the main uses of papers.

It would be good to adjust the defaults so that:

• papers install --local defaults to local bibliography and filesdir
• default to an existing bibliography if any is found

To make it more useful, it might also be good to have a recursive, upward search like git