mark-yank-urls: Ability to define handlers per protocol

This change introduces the concept of "handler", a command that
handles a specific protocol. E.g., x-www-browser for handling http,
https and ftp (default). Pressing enter when marking URLs with
other schemes will no op. This still leaves us with the possibility
to yank the URL to the clipboard.
Support scheme less www domains

Guess scheme to be "http" if there is no real scheme in the URL,
but the domain is "www.*".