Bazel Support

It'd be nice if the Arduino compile had Bazel (http://bazel.build) integration.

Something like:

```
arduino_cc_binary(
name = "example",
srcs = ["example.ino"],
board = "@io_github_arduino_arduino//boards:esp8266",
libraries = ["rs232"],
)
```
This would download the board definition, arduino compiler, and required libraries and build them to bazel-bin/example.bin

Start by investigating how the Arduino compile currently obtains board definitions, the compiler, and libraries, then compare those needs with the proposed Bazel target. The example target in the issue is the requested starting point; done should mean it can build the example sketch into bazel-bin/example.bin.

Written by the indexing model from the issue text.