Fallback preference to simulate copy and paste in custom editors

[1ec5]

When support for a particular custom editor breaks, it often takes awhile for a fix to come out and be approved by the AMO editors. During this time, it would greatly comfort users if they had access to a fallback mode that simulates copy and paste (like the Google Docs support currently does) instead of integrating so tightly and fragilely with the custom editor.

The fallback mode would be a preference, off by default; users would find this preference on their own or we’d direct users to it when they complain loudly enough about broken support.

See #96 and #105 for additional background information.