tagPreRender event in tags plugin

Adding a tagPreRender event in tags plugin that triggers just before a tag is going to be rendered, allowing ability to change the value of the tag to be displayed.

This is very useful in cases where you cannot change the value of the tag at the source because: a) Its value is shared with some other plugin, say - suggestions or autocomplete, b) ItemManager also cannot be used as you want to do this special treatment of the value just for tags.

Example - In the autocomplete, you want the suggestions to appear as "Name: Description", but when the user clicks on it and a tag is generated, it should only have "Name" in the tag.