cleanExtraTags (and cleanExtraBrTags) Dom Modifiers don't properly account for multiple selector matches

Here's a further change that is possibly a bit more controversial, but I think it's in the spirit of what this method is meant to do. This updates the previous patch to strip ALL of the instances of the given tag from the beginning/end of the markup. This is how I expected the function to work given the fact that "Tags" is plural in "cleanExtraTags" (it's not "cleanExtraTag").

So if you have something like:

<p>Text goes here.<br><br><br><br></p>

Then ALL of the
tags will be stripped when you run that through cleanExtraBrTags with this patch.

@azinck, thank you so much for your work on this and the contribution. I am sorry for letting this sit so long without responding.

I think your approach is right. I need a good way to test this and at the moment I don't have a good existing migration to try it out with. I am going to need a little time to test this out.