This will be used in youtube-dl integration, to tell when a html page has
been downloaded by addurl, in which case it is worth running youtube-dl
to see if it can extract media from it.
tagsoup is an almost free dependency, because yesod depends on it.
So, this only really adds a dep when git-annex is built without the
webapp.
I'd like this to as closely as possible match how browsers decide if a
page is html or not. Unfortunately, that is fairly heuristic, in order
to support malformed html. And, we don't want to falsely detect
something as html just because it has something that looks like a html
tag embedded somewhere in it. Probably any major video hosting site is
going to be serving html documents that at least start with a <html>
tag, so requiring that or a DOCTYPE should be good enough.
This commit was sponsored by Jeff Goeke-Smith on Patreon.
