Add support to output checkstyle xml

[rahulsom]

jscs supports -r checkstyle to generate checkstyle xml output. That is really useful when used with a CI server like jenkins.

[nighca]

Thanks for feedback.

We'll take it into consideration.

[chriswong]

https://github.com/ecomfe/fecs/wiki/CLI#options-1

We do this work in fecs：

$ fecs check xxx.html --format checkstyle