Add option for link target

Just added the None option. Very useful feature.

Didn't do the tests.

Actually breaks W3C compliance. When not filled, it is adding an empty target attribute which is not valid.

target=""

I am working on this issue.

@flocondetoile I worked here and checked target attribute on the link. If we use none option for target then an empty target attribute comes with link tag. Can we create a separate Field Formatter for this?
cc @maintainers

A field formatter is not in the scope of this module. It's not the right way to support this.