Refine frame handler logic for frame name and geometry

[shipmints]

Satisfies #46 and #52.

Change logic to explicitly test for bufferlo--bookmark-set-loading and the explicit values of bufferlo-bookmark-frame-load-make-frame rather than assuming bufferlo-bookmark-frame-load-make-frame will always be nil while loading a set.

Add bufferlo-bookmark-frame-load-make-frame 'reuse-restore-geometry to accommodate the case where a user does not want a new frame but does want to restore a frame bookmark's geometry.

Update documentation.