Add CSS variable to browser

Documentation at
https://developer.mozilla.org/en-US/docs/Web/API/CSS

Specified at
http://dev.w3.org/csswg/css-conditional/#the-css-interface
http://dev.w3.org/csswg/cssom/#the-css.escape%28%29-method
Merge pull request #1899 from Rob--W/variable-CSS

Add CSS variable to browser
Add mock-stdin as devDependency

It helps test the stdin file input in the cli suite.
Add tests for gh-1880
Add .jshintrc overrides handling to stdin processing

The new .jshintrc overrides were not being handled by
SublimeLinter-jshint because the --filename flag was not
being passed to the lint() function.  This has been fixed.

Closes #1880
Move `useStdin` tests into own suite

Closes #1900