Add notNull modifier

[lukescott]

Fixes #57

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[jharris4]

I was just thinking about creating a PR for almost this exact same feature, except I was thinking of calling it isDefined instead of notNull.

Basically, I have a case where I want to allow null but disallow undefined.

[lukescott]

Closing in favor of #90

[jharris4]

@lukescott thanks for the initial work on this, hope you don't feel like I stepped on your toes making a similar PR ;-)