Add an "AlwaysUnderlined" link variant

[bitspittle]

Right now, the default behavior is to only underline if the cursor moves over the link. Some people might want always underlined behavior, and while it's easy enough to let them write it, it's also trivial for us to provide a variant for them.

[bitspittle]

Assigning to myself now since it's trivial to do.

[DennisTsar]

Fixed in #342