New: Allow BoxAnnotations to be instantiated independently and passed into Preview as an option

[pramodsum]

Dependent on box/box-content-preview#539

This will allow a developer to instantiate annotations outside of Preview like this:

const options = {
    Image: {
        enabled: true
    },

    // Document/Presentation Annotators are the same so the config is simplified here
    Document: {
        enabledTypes: ['highlight', 'draw']
    }
};


/* global BoxAnnotations */
const boxAnnotations = new BoxAnnotations(options);

var preview = new Box.Preview();
preview.show(FILE_ID, ACCESS_TOKEN, {
    container: '.preview-container',
    showDownload: true,
    boxAnnotations
});

[boxcla]

Verified that @pramodsum has signed the CLA. Thanks for the pull request!

[JustinHoldstock]

Guard for whether or not annotator exists

[pramodsum]

re-working how these configs are set