Plugin system and a batsrc

Adds a load_plugin function to share functionality across projects.
For a plugin `foo` its default path would be `~/.bats/plugins/foo/foo.plugin.bats`
This file gets sourced - if people want to build more complex stuff they can use their `foo` to do it clean and they should souce the files inside of `foo.plugin.bats`.

Also adds sourcing of a ~/.batsrc file, which is done just before execution starts in bats-exec-test. People can mess with bats their in every way they want, I for now would use it to load plugins.

If you think this is something that can be useful, I'd add tests and update the readme.