Fixes #812 - Activate hover effect on link

Add an unmapped command for a mode to trigger mouseover effect on a link. There are a number of caveats here:

1. I think the link hints should be filtered for this functionality to only those elements that have a mouseover event handler, but because there doesn't seem to be any way in the DOM API to retrieve the list of event handlers for an element, I am not sure this is possible.

2. For the same reason as #1, there may be some elements on the page that cannot be triggered using this function (for instance, amazon.com uses li elements for their sidenav menu items)

3. I am not aware of any way to detect whether or not the mouseover event is active on an element. Ideally I would like to have the function trigger the mouseout event if the mouseover event is active but I was unable to figure out a way to do this.

4. There doesn't seem to be a way for Javascript to move the mouse cursor to the element, only to trigger the effect. This makes sense as I imagine pages could get really annoying if they could move the mouse cursor.

In light of all this, feel free to reject the pull request and/or offer any suggestions on how to improve it. I'd be happy to work on this more if anyone has any tips.