Allow using an arbitrary program instead of urlview, with command-line args

[Boruch-Baum]

Many people prefer urlscan to urlview, although urlscan can be agonizingly slow. The PR also sneaks in "better"(?) error messages.

[bhean]

That's exactly what I was looking for! I need to use a custom "URL opener" and this is the best way to do it. Please, review&merge this PR! Thanks!!