Add option to place extension menu items at top.

This addresses #15, though this functionality could be implemented in a
few different ways and is worth discussing.
Add Menu extensions example.

The example is taken straight from the docs.
Allow only a group to be exported.

Implements the request in #84, though I'm unsure if placement as
submenu items are the best UX.
Add test for exporting groups.
Update doc comment to specify optional parameter.
Merge pull request #166 from mminer/positionable-menu-extensions

Implement positionable menu extensions
Merge pull request #168 from mminer/export-group

Add ability to export group