Support for tags on CLI

Created by: lailsonbm

Filters are one of my favorite things on RSpec 2, specially because I don't use Cucumber. But is very cumbersome always have to set filter_run_including and filter_run_excluding on the spec_helper file, so I implemented a simple support for tags on the CLI. It is just a subset of the described by David in http://github.com/rspec/rspec-core/issues/#issue/37 (actually just the section "keys and/or key/value pairs"), but I think it covers most cases. To see exactly what was implemented, refer to the Cucumber feature http://github.com/lailsonbm/rspec-core/blob/b4501b6d5e9d7ddc930d5d4cec05f1cc0d201897/features/command_line/tag.feature.