Add support for hints in package.nls.json

[Colengms]

Addresses: #32

Hints can be added to package.nls.json in the following format:

    "key": {
        "message": "<message>",
        "comment": [ "<hint1>", "<hint2>" ]
    },

There was already support in place for objects with a message field in package.nls.json, so this should not break compatibility with that.

There was existing support in addFile() for a comment array to be pass in via the key (though, using message would have made more sense to me). This looks to be used by existing support for hints in calls to localize(). I updated createXlfFiles() to leverage this existing support to store hints/comments from package.nls.json in the key.
I also updated addFile() to use only the key string value itself for comparisons with existingKeys.

[Colengms]

@dbaeumer Could you review this? Thanks!

[dbaeumer]

I published 4.0.0-next.1. Since the code was quite outdated I did a cleanup and added the feature afterwards by hand based on what you have proposed. I also added tests.