feat($Gruntfile): Command-line args for tests

You can pass in `autoWatch`, `singleRun`, `reporters`, and/or
`port` as command-line arguments with the format --<key>=<val> on
the command-line. Now the `grunt test:<target>` command will accept
these values and you can modify them in the shell!

Ex:
    # this will trigger a CI run of the respective browers
    grunt test:PhantomJS,Chrome,Firefox --singleRun=true --autoWatch=true