Add operatorfunc function, new mapping, and cleanup README

This lets us do things like `,<ip` to export a paragraph.  I've really missed having this, and now that I've tried it out, it's pretty slick.

Also I fixed up the `README`, including the rather glaring omission of our syncopate-specific configuration options.
I found this wording slightly confusing, but I'm having a hard time rewording it. How about something like:

`,<>` calls `:SyncopateExportToClipboard` on the whole buffer (or your selection in visual mode).