Support user-supplied metadata for #its

Created by: AlphaHydrae

I'm working on a test analysis tool which relies heavily on user-supplied metadata such as this:

it "should work", :meta => :data do
  # ...
end

There's currently no way to add metadata to an example defined with its aside from wrapping it in an example group:

subject{ "foo" }
describe "group for metadata", :meta => :data do
  its(:to_s){ should eq("foo") }
end

This is very verbose when you have many its examples. With the changes in this commit, its forwards additional arguments to its internal describe call, allowing this:

subject{ "foo" }
its(:to_s, :meta => :data){ should eq("foo") }