fix(rxPage): Ensure document title updates if `scope.title` changes

Closes #447

Previously we would only call `rxPageTitle.setTitle` one time,
when `rx-page` was loaded. If the expression we pass to the `title`
attribute of `rxPage` changes, we weren't re-calling `rxPageTitle.setTitle`.

This commit puts a watch on `title` and re-calls `setTitle` on change