Handle IPv6 hostmask in parse()

Signed-off-by: James McCoy <vega.james@gmail.com>
Switch to vimhelp.appspot.com

As the tag file being parsed is extracted from Mercurial, using
vimhelp.appspot.com is more likely to work than vimdoc.sourceforge.net.
The former is updated automatically as the Mercurial repo is updated.

Signed-off-by: James McCoy <vega.james@gmail.com>