Make syncopate work with motions

Added a new public function, syncopate#ClipboardOperatorfunc(), and a
functions section in documentation.

Changed the mapping prefix to `<Leader><`, and a new mapping with the
bare prefix to work with operators.

Updated the plugins section in the README.

Fixes #28.
Mention syncopate-specific configs

This was a longstanding omission, which has grown more unfortunate as
the number of syncopate-specific options has increased!
Update CHANGELOG for operatorfunc