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