When the autofocus parameter is set, check that
the specified element exists in the DOM and
check that the specified ID is unique. If there
is not exactly 1 element with the given ID, warn
the user about unexpected behavior.
Users can provide the id of an element to focus on when the
slide is displayed. While HTML5 has an autofocus parameter
it is not universal and not all browsers support it. This
solution should work across browsers and is more in keeping
with the jsPsych object style.
Issue #1062
