Add link to GitHub Issues page for support in seedtool --help

In `seedtool --help`, instead of sending email to my blockchain commons email address, add link to GitHub Issues instead. The README.md should do the same if it doesn't already.

Extra credit if you can figure out why some links in Apple Terminal.app are right-clickable and some are not — love to add that if we can figure out a way that doesn't break linux cli.

-- Christopher Allen