feat: required properties are supplied via a constructor function

[ColinEberhardt]

@OiNutter based on your suggestions on this PR #50 - this is an alternative implementation where constructors are used for required properties.

The debate about interfaces / classes aside (#51), I do think this is a cleaner way to handle required properties.

[OiNutter]

Great, happy it was a sensible suggestion!

Not that familiar with the code so my approving the request is meaningless but what you're doing looks sensible.

Quick one just from looking at it though, have you considered using .hbs file extensions instead of .handlebars? Didn't know if there'd been a conscious choice to go this way, or if you weren't aware of the .hbs extension? Just thought it might save some typing 😄