Add the ability to supply a custom render function

[rsnyman]

An application I'm working needs to render custom components, so this PR implements an optional custom render function.

BTW, I added a test, but I could never get the tests to run, there were numerous errors outside of the actual tests.

[rsnyman]

@bhubr Sorry I didn't get back to this sooner, I didn't see your replies till today! I've fixed the issues you requested and pushed up a new version.

[bhubr]

Thanks for your work, merging it right now!

[bhubr]

@rsnyman I forgot to suggest updating the version number, so I'll do it myself, and mention you in the contributors in the readme (and republish the package on npm)

[rsnyman]

Thank you!