Create a default notmuch-show-hook that highlights URLs and uses word-wrap

I created the notmuch-show-hook precisely so I could add these two
options, but I suspect most people will want them, so I just made them
the default. If you don't want them, you can use remove-hook to get
rid of this.

Signed-off-by: Keith Packard <keithp@keithp.com>