STDIN Support

[steelbrain]

We are using gjslint in our atom package and whenever we want to lint a file that hasn't been saved to disk yet, we have to save it to a temp location to be able to use it with gjslint. It would be nice to have it accept files via stdin like other linters do. They often have a parameter such as -s/--stdin for this kind-of purposes. The linters who look for rc or configuration files using that file path take additional parameters like --stdin-filepath.

Ref: AtomLinter/linter-gjslint#16