Handle a single pair of ()/[] around URLs

now handles:
(site.com)
[site.com]
(site.com]
(blah site.com)
(site.com blah)
(site.com/(a-valid-url-in-matching-brackets) blah)

still does not handle:
((site.com))
(site.com/a-single-ending-bracket-in-url) blah)