Add check against missing/mispelled plugins.
Add handling for fuzzy EE booleans.
Add support for forcing strict comparison.

This allows using things that look like EE fuzzy booleans, without
the plugin incorrectly using the fuzzy comparisons.
Fix potential bug when no method defined.

Was using too loose a comparison, that might have let '' be
considered a method name, and result in an invalid call to the
plugin class.
Add method existence checking too.
Be consistent with warning format.