support for more than one tag simultaneously

Created by: rodrigues

I did some specs for it, but this part of code changed just now in rspec-core.

Guys, I'd be glad to know your opinion on it, also any improvements I need to do in order to enable this feature in rspec.

We have a heavy integration specs project that motivated me to do this change. Some contexts are slow and we don't want to run them all the time. Other just fail, and since they fail because of an external dependency, they're not pending in our side. We'd like to use multiple tags to exclude the right ones, and enable the use of wip, for instance.