Allow tags placeholder to be configured

[chrisgid]

I think it would be useful to be able to configure the placeholder text for the Tags property editor.

I have a situation where I have two instances of the editor one for 'Tags' and one for 'Categories' and the placeholder text doesn't quite make sense for the 'Categories' one. I think it would be useful to customize this so it says "Type to add categories"

I'm happy to submit a pull request for this!