Implement the --os command line switch

The --os command-line switch didn't appear to be implemented.

The OS needs to be specified as name:version, e.g. "Windows:XP" or "OS X:Mountain Lion". Which is a bit clumsy looking, but it fits with the browser version style and it sort of matches the output from the "browsers" command.

Perhaps it could be supported with the "Name (Version)" style. If you like the sound of that I would be happy to implement that for both OS and browser.

Browseroverflow didn't help with converting browser versions to the Browserstack style when all of the parameters were passed. So chrome:20 didn't match chrome:20.0 and raised an error. I have submitted a pull request to browseroverflow that addresses that issue. https://github.com/airportyh/browseroverflow/pull/2