Support user-supplied metadata for #its

I'm working on a test analysis tool which relies heavily on user-supplied metadata such as this:

```rb
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:

```rb
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:

```rb
subject{ "foo" }
its(:to_s, :meta => :data){ should eq("foo") }
```