Allow per-instance setting of "editorStore" for SirTrevor.Editor

Hi,

I recently had a need for per-instance setting of SirTrevor.editorStore, so I added support for setting an "editorStore" property on the "options" object given to the SirTrevor.Editor constructor.

I'd like to have it in upstream too -- hence this pull request :).

(I assumed that I wouldn't need to include "sir-trevor.js" in the pull req. since that's a generated file, but please let me know if I *do* need to include it in the pull req.)

Regards,