Add setting to leave links with target unchanged.

The current extlink module has a setting to take all external links found, and set the target attribute to "_blank". I like this setting, but I want to give editors the ability to specify where they want the link to open, if necessary. So, if I know I want a link to open in _self and I put that in my HTML, I think extlink should leave it alone and not change _self to _blank.

I built this in a similar fashion to 2874092 which appears to already have the maintainers blessing. I used similar named variables and form behavior. The general behavior here is that when this setting is checked, the links are filtered out of the list of links that have target=_blank set. It's pretty straight forward.

Note: this does not update the minified file, as I also opened 2929552 to remove that file, per discussion on other threads I found.