add ability for user to extend built in types

[austinpray]

User should be able to provide a map of their custom types to override the built in types: https://github.com/austinpray/asset-builder/blob/db6d6739b4324becde7326c8afedb3941ced8e00/lib/types.json

Related: https://github.com/austinpray/asset-builder/issues/33

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[kalenjohnson]

I've looked through the source. trying to figure out where custom types should go.

I don't think a new types.json should be required. Is there a reason that the default types are in a json file anyways? Seems like the defaults could be added somewhere like this: https://github.com/kalenjohnson/asset-builder/blob/master/lib/processManifest.js#L19-22

And then we could add an optional files override to manifest.json.

I played with the code a bit, but if the files are actually in the manifest object, then you would need to pass the full manifest object here, but then I think that's starting to fight the structure you have set up. Probably need to set up another prototype function.

[austinpray]

@kalenjohnson the first step is to write some failing tests ;)