Adding ability to merge tags

[pichot]

Hello!

Wanted to put this here for your thoughts. A client project needed the ability to merge tags, instead of just renaming them. I thought it might be a useful feature for others since Craft allows tags to have the same string.

The merge implementation was straightforward thanks to a merge function on ElementsService. But I did have trouble getting the dropdown menu of tags to work as expected. The issue came down to the fact that I add tags to a list with jQuery when the menu is displayed. I couldn't find how to get Craft's click handlers to stick, and instead had to write my own.

I'd appreciate your thoughts on improvements, and if you think this is something you'd be interested in merging. Thanks!