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.

首先查看 Spotless 现有的插件结构和 spotless-lib 入口点，然后比较 Bazel 的 Skylark 扩展、shell 命令和二进制集成选项。完成标准是提供有文档说明的 Bazel 集成，使其能够调用 spotless-lib 或 Java wrapper，但该 issue 没有指出应从哪些文件或测试开始。

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