An option to search only OS X or iOS documentation

Hi!

I use Dash for searching documentation for OS X, iOS, Ruby, Rails and CocoaPods plugins.

It bugged me that when I was working on my Mac app, alt-clicking on method names searched all the documentation, not just OSX's (it takes more time and often there's a bunch of, say, Ruby stuff I'm not interested in). Likewise, when I'm learning iOS dev, I'm usually only interested in classes and methods from iOS docset.

So I changed your awesome plugin a little bit and added an option in Edit menu to only search in OS X or iOS docsets:

![Zrzut ekranu 2013-01-16 o 11 08 12](https://f.cloud.github.com/assets/183747/70852/9559d898-5fc5-11e2-8165-9e74dc6d4a3c.png)

I think more people use Dash that way and would find it useful to only search a certain docset.