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. It's possible to override the global `.batsrc` in the home directory with a local `.batsrc`, to be placed inside the `test` directory (I guess it's a good convention to place all tests in such a directory anyway)

If you think this is something that can be useful, I'd add tests and update the readme.

Edit:
If someone wants to try it out with the very first bast plugin ever written, check here: https://github.com/LFDM/bats_matchers - just clone the repo into your `.bats/plugins` dir.
:) It's rather uneventful so far.