Add option to set a custom view for notifications.

This is just a preview PR to get some preliminary feedback before I submit this upstream. @djsell, could you take a look?

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This changeset enables the creation of custom layouts for notifications. I had to make a few things public that weren't before, namely the option defaults system and some parts of `CRToastView`, but hopefully the extra functionality is worth this cost.
Yeah :unamused:. Look [a few lines below here](https://github.com/mkantor/CRToast/pull/1/files#diff-5f3010153047668460039f76eed8df7cR213) for examples.