Add support for running validation in parallel

[johnpeb]

JSHint is relatively computationally expensive and most developer computers have multiple cores.

JSHint currently runs in a single thread but it should take advantage of all cores.

[Marco-Sulla]

I would contribute to solve this problem. JSHint on Eclipse takes me too much time. Where should I start to look?

[syskin345]

I implemented it in my fork, enjoy.