Add Load.options() for hierchical construction of loader options

Add Load.options() for hierchical construction of loader options

Added new loader option method :meth:`.Load.options` which allows loader
options to be constructed hierarchically, so that many sub-options can be
applied to a particular path without needing to call :func:`.defaultload`
many times.  Thanks to Alessio Bogon for the idea.

Also applies a large pass to the loader option documentation which
needed improvement.

Fixes: #4736
Change-Id: I93c453e30a20c074f27e87cf7e95b13dd3f2b494