[java] allow configurations to be used with a browser string value

[titusfortner]

I don't know how I feel about this. People doing parameterized platform execution can not currently use the new syntax. This would allow them to do it, but I'm not sure if it is feature creep.

Somewhat tangentially I'm now wondering if we should have put all the methods into SauceOptionsBuilder class instead of having them on SauceOptions directly. I can't remember if we considered this and rejected it or just overlooked it:

SauceOptions options = SauceOptionsBuilder.chrome().setChromeThingOne().setChromeThingTwo().build();
SauceOptions options = SauceOptionsBuilder.usingBrowser("chrome").setChromeThingOne().setChromeThingTwo().build();