rustc can produce errors in json format for easy machine processing

[oli-obk]

This can be enabled by setting the environment variable RUSTFLAGS to -Zunstable-options --error-format=json.

@noseglid what do you think about adding a "generate_matches" callback to the settings structure that takes the captured stdout string as an argument and produces match-structs? This would neatly allow this feature to be implemented.

[noseglid]

Maybe errorMatch could be a function optionally, which receives stdout and stderr and should return match errors. Not a bad idea.

It could also be useful with the .atom-build.js, where functions can be defined.

[noseglid]

This was fixed by #29