This change makes it clear that the v2 import path
is github.com/russross/blackfriday/v2, and updates
various links accordingly.

See https://github.com/russross/blackfriday/issues/587#issuecomment-703393820 for details.

This change also converges the README for v1 and v2
to be consistent, as they've started to drift apart.

For #587.

GitHub-Pull-Request: #675