text: Title: add an alternative to find_by_text that looks at all inner text
  Body: The current implementation of `find_by_text` uses the following xpath: `//*[text()="some text"]`, which only looks at the first text node within an element. This makes it difficult to query for elements with text split across multiple text nodes.
  
  I think it would be useful to add an alternative that acts more like `element.textContent()`, querying against all inner text of an element rather than its first text node. This can be done with the following xpath: `//*[.="some text"]`
  
  It could be called `find_by_text_content` or `find_by_inner_text` or something along those lines.
  
  I'm happy to do the implementation and make a pull request if people agree that there's a need / desire for this