Allow selection of text in a descendant element

[z0u]

This adds the ability to select a descendant, instead of selecting all text in the clicked element.

<div select-text>
  Clicking this div selects only the text in
  <span select-text-target>this span element</span>
</div>

Also updated some docs and added a unit test for the new behaviour.

[z0u]

Removed dependency on jQuery. [...]

Ignore this: it's left over from a commit that I squashed.