Allow options on Mash#load

[dblock]

A replacement for #473. The only users affected by this change would be those that have rolled a custom parser, vs. #473 that affects everyone who calls Mash#load.

• Preserves backwards compatibility and doesn't change the behavior of Mash#load.
• Allows passing options through, so one can Mash.load(file, whitelist_classes: [Symbol]) which was the desired effect of Enable load symbol class #473, and can specify other options such as aliases.
• Documents the feature and the interface change for custom parsers.

[dblock]

@michaelherold Care to CR/squash/merge? Thanks.

[michaelherold]

I'm good with this. The tests for the Psych::DisallowedClass were a little unclear but that's due to the nature of the stdlib implementation. I can't fault you for that!