Implemented ability to pass arguments to the script

Hi guys!

Hope that I didn't do anything criminal with that :) Please comment if you have any objections or suggestions how to improve that feature.

I thought that using PowerArgs might be an excellent choice here (but sure, no support for dynamic in Roslyn CTP is a pity :( ). I don't know whether extending ScriptHost was a good idea, but I haven't found any other non-clunky way, to advertise this feature to the script. 

See usage example here https://github.com/yevhen/scriptcs/tree/script_args#passing-arguments-to-the-script

P.S. Eager to hear your comments !

Thanks,
Yevhen