Add plugin support for Bazel

[Bazel](https://bazel.build/) is something which has interested me for a while now, so writing a Spotless plugin for it may prove to be a good way to get my feet wet with it. However I'm limited for time with my studies and I suspect I won't be able to work on something this big until late May 2017 or later.

Here's what I currently understand about Bazel with regards to plugin-writing:
1. Bazel uses a subset of Python called [Skylark](https://bazel.build/versions/master/docs/skylark/concepts.html#syntax) for writing [_extensions_](https://docs.bazel.build/versions/master/skylark/index.html) (Bazel's terminology for plugins AFAICT). So it may very well be that Spotless would need to be ported to Skylark first, which isn't ideal.
2. There seem to be ways of integrating [shell scripts](https://bazel.build/versions/master/docs/skylark/cookbook.html#simple-shell-command) and [binaries](https://bazel.build/versions/master/docs/skylark/cookbook.html#execute-bin) as extensions into Bazel. I'm hoping that we can use one of these options or some another option to call `spotless-lib` (or a Java binary wrapper around it) without porting it to Skylark.