Add paranoia about ClassLoader in ConfigParseOptions

Fixes #155. Really this is kind of a mess; having overloads
with both ClassLoader and ConfigParseOptions parameters, when
the parse options may include a class loader, confuses the
heck out of things.

Trying to make it so that the ConfigParseOptions variants are
the canonical version, and the ones with a loader just immediately
set the loader on the parse options and call the variant with no
loader parameter.

Anyway ... when it fails this patch should at least make it much
clearer what went wrong.