Option to require new window

[timkelty]

For a field that you want to require to be external, it would be great if you could not only "Allow links to open in new window", but require them to.

Related: #68

In the settings, perhaps both "Allow links…" and "Allow custom link test" could have nested "required" checkboxes appear beneath them when checked.

On the field front-end, the "open in new window" could be checked and dimmed out (read-only).

[robmcfadden]

It would also be nice to still allow a user to make the selection but default to opening in a new window.