Expose JSAPI as a factory method

Hello.

I would like to use SVGO as a pipeline for optimization and transformation with my own plugins. All content items must be instances of the JSAPI class to not break the chain of the existing plugins. Unfortunately, consumers of the SVGO module don’t have direct access to that class.

So, I propose to add new `createContentItem` method to the public SVGO API. It allows to create new items without exposing inner class.