Add feature to click spans inside a TextView

As asked and resolved in [this StackOverflow thread](https://stackoverflow.com/a/40524247), it would be great to have a method to click spans inside TextViews, which is not directly trivial with Espresso only.

I suggest a method like `clickSpan(viewId, spanText)`

Start by reading the linked Stack Overflow answer and locating the repository's Espresso and TextView interaction entry points; the issue names no file or test. Done means providing a clickSpan(viewId, spanText)-style method that can click the requested span inside a TextView, with tests demonstrating that interaction.

Escrita pelo modelo de indexação a partir do texto da issue.