Add fill_form_and_submit

The pattern of filling a form then immediately submitting seems to be the most common interaction, and the more I use this the more I think they should be combined.

I'd like to keep both helpers separate as well in case there is something that needs to be done between the interactions, so this would be an addition to the public interface.
is the string the css selector? Perhaps specify that, while we're adding stuff here?

eg:

    The usual workaround is to use the CSS selector (as a string) as the attribute key.