Add support for returning promise like objects from load.

[topaxi]

This PR allows the load setting to return promises instead of calling the provided callback.

[topaxi]

If this feature is ever considered to be added to selectize, I will add tests and documentation for it, so this could be merged. Otherwise, feel free to close this PR.

[Ramblurr]

This is great! I also wish the render callbacks handled promises.

[joallard]

Is this an api-breaking change?

[topaxi]

This shouldn't be breaking, as long as nobody is returning a promise from the load function and calling the done callback, which is very unlikely.

[joallard]

I'm prepared to accept it. Can you add some test coverage?

[topaxi]

Rebased againts master and added some basic tests.

[topaxi]

Just going through a lot of my open PRs on Github. Anything left to do here from my side? Feel free to close this if you do not want to extend the selectize API with the proposed changes :)

[github-actions]

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