Merge with custom_chewie plugin

Let's merge back the features of the [custom_chewie](https://pub.dartlang.org/packages/custom_chewie) plugin. Unless there are very good technical reasons I don't see the point of having two very similar plugins that even have almost the same name.

We recently did a major refactor of the code which should make it much easier to customise things. https://github.com/brianegan/chewie/issues/100 tracks the effort to make customising more end-user friendly.

@bostrot, what do you think? Would you want to collaborate on this?

Start by reading the custom_chewie package linked in the issue and issue #100, then compare its features with the current chewie plugin after the major refactor. Confirm the desired scope with the maintainers and custom_chewie author; done means an agreed, technically justified integration rather than two similar plugins.

Written by the indexing model from the issue text.