Add support for local binary repository

[danpat]

Following the pattern of tools like ivy repositories, apt-get PPA's, etc, mason should have the option to look for binaries in local directories, rather than only looking at GitHub/S3.

This would be useful when making multi-project changes (e.g. new feature in mapnik, requires updates to node-mapnik bindings, and then a change on Mapbox Studio), meaning that nothing special would need to be done as new code flows up the chain, it could be as simple as mason publish --local in the downstream project, then mason install <package> <localversion> in the upstream project.

[springmeyer]

:+1: