add --no-robots commandline flag

[anarcat]

While this flag can be abused, it seems to me like a legitimate use
case that you want to check a fairly small document for mistakes,
which includes references to a website which has a robots.txt that
denies all robots. It turns out that most websites do not add a
permission for LinkCheck to use their site, and some sites, like the
Debian BTS for example, are very hostile with bots in general.

Between me using linkcheck and me using my web browser to check those
links one by one, there is not a big difference. In fact, using
linkcheck may be better for the website because it will use HEAD
requests instead of a GET, and will not fetch all page elements
(javascript, images, etc) which can often be fairly big.

Besides, hostile users will patch the software themselves: it took me
only a few minutes to disable the check, and a few more to make that
into a proper patch.

By forcing robots.txt without any other option, we are hurting our
good users and not keeping hostile users from doing harm.

The patch is still incomplete, but works. It lacks: documentation and
unit tests.

Closes: #508

[anarcat]

this patch was merged in master in the new linkcheck organization.