terminal: add internal API to format URLs for display in capable terminals

terminal: add internal API to format URLs for display in capable terminals

Newer terminals (in particular gnome-terminal) understand special escape
sequence for formatting clickable links. Let's support that to make our
tool output more clickable where that's appropriate.

For details see this:

https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda

The one big issue is that 'less' currently doesn't grok this, and
doesn't ignore sequence like regular terminal implementations do if they
don't support it. Hence for now, let's disable URL output if a pager is
used. We should revisit that though as soon as less added support for it
and enough time passed for it to enter various distributions.