Enable parsing command line arguments in the test runner

[jrjrguo]

My recent PR #108 enables the test runner to take in an optional dictionary with NUnit test context parameters along with the test assembly. It solves the problem of no being able to set test context parameters, but the users have to make the dictionaries and pass it to NUnit.Xamarin. I think a more direct way is to enable the test runner parsing command line parameters. In addition, it would be better if users can set the test options (AutoRun, ResultFilePath, tcp parameters...) from command line as well.