Add ability to optionally select a specific test to run.

I wanted to provide a way to only run a specific test in a test file that didn't break how the plugin already worked.  So now, if you have highlighted some text then you use the command to run mxunit tests in the browser it will append the url argument &testMethod=foo where foo is the highlighted text.

This change works with both the command pallet and the context menu.  If no text is highlighted the standard behavior continues (all tests in the open file are executed).