vimperator: setup editor via map override, not autocmd

This does not trigger an autocmd on every page and allows for better
customization.

Based on the idea from:
http://ingo-karkat.de/blog/2012/05/22/Pentadactyl%20set%20filetype%20in%20external%20Vim%20editor%20based%20on%20URL.html
vimp: override y mapping to use the encoded url

Ref: http://code.google.com/p/vimperator-labs/issues/detail?id=797
vimp: minor fixes