Allow Get Element Text algorithm to be set via a capability

[shs96c]

At the Lisbon F2F we agreed to use the atom from the Selenium project for Get Element Text as this is what users expect and because there wasn't a stable alternative (the inner text algorithm not being equivalent)

We also agreed it would be desirable to be able to use innerText once it was available in all browsers and stable. This feature should be exposed to the user via a capability. I'd suggest text rendering with the possible values of atom and inner text (with the default being atom to preserve compatibility) would be a reasonable key name and values.

We need to ensure that element retrieval algorithms uses the same algorithm as Get Element Text (which I think they do) when this change happens.