tidy: exempt URLs from the line length restriction

tidy: exempt URLs from the line length restriction

The length of a URL is usually not under our control, and Markdown
provides no way to split a URL in the middle.  Therefore, comment
lines consisting _solely_ of a URL (possibly with a Markdown link
label in front) should be exempt from the line-length restriction.

Inline hyperlink destinations ( `[foo](http://...)` notation ) are
_not_ exempt, because it is my arrogant opinion that long lines of
that type make the source text illegible.

The patch adds dependencies on the `regex` and `lazy_static` crates
to the tidy utility.  This _appears_ to Just Work, but if you would
rather not have that dependency I am willing to provide a hand-written
parser instead.