Add links from options -> arguments and back again

This helps draw the distinction between arguments and options, which
probably isn't familiar to people coming from argparse.
Merge pull request #182 from quodlibetor/doc-hyperlinks

Add links from options -> arguments and back again
Restore the Click 1.0 help handling.
This is 2.3
Merge branch '2.x-maintenance'
Merge branch 'master' of https://github.com/mitsuhiko/click
Tab-completion for prompts