Add extension DSL for elements about bowser

[ajjahn]

I like this approach. I greatly appreciate bowser's small foot print, but very regularly find the 'method_missing magic' doesn't cover my app's needs. I usually resort to monkey patching in additional methods. It would be great to be able to just require in the functionality needed, and nothing more.

[jgaskins]

I'll add a PR this weekend for this with the proof of concept I've got. It works pretty well. It still lets you rely on the default method_missing for properties that return primitive values and keeps the payload small.