Add CLI runner option for choosing which directory to save coverage reports in

Hi all. This works pretty much as described, and you to do the following to chose where coverage reports are saved.

    guard-jasmine --coverage-html --coverage-html-directory=./my/coverage/directory

Let me know if there are any issues with the code. We're using guard-jasmine as part of a large internal startup and would like to be able to keep our ruby and js specs in certain folders. We're currently relying on an ugly rake task to find and move them all.

Thanks for all your hard work! :grinning: