Adds Global and Local Configuration Variables

See [this RFC](https://forums.tessel.io/t/config-variables/1331) for the intended functionality.

@natevw can you check it over?
This makes it very easy to accidentally commit API keys if you use the library from a `npm link` git clone.

Config files don't go in the package directory. They go in a platform-dependent location under the user's home directory -- `~/.config/` on Linux, `%APPDATA%` on Windows, `~/Library/Application Support` on OSX. There's probably an npm package for that...