Add --coverage_html support to cli

Unless I'm missing something (I'm new to guard-jasmine), there was no way to get html coverage when running shell-level guard-jasmine outside of guard.  So I added  --coverage_html to the CLI options, and passed it along.

The only extra liberty I took is to have --coverage_html imply --coverage.  Seems reasonable to me that 
   $ guard-jasmine --coverage_html
should work, rather than needing
   $ guard-jasmine --coverage --coverage_html

Thanks much for putting all this together!