Use extras to reduce size of dependencies

[skbaur]

As contexual has a lot of depedencies that are not necessary for all use-cases, would it be possible to split the depedencies in different sets of extra requirements (similar to the strategy of https://hanxiao.io/2019/11/07/A-Better-Practice-for-Managing-extras-require-Dependencies-in-Python/ ). This would help reduce the size of the installed depedencies and also reduce potential version conflicts.

[wangjin1024]

@skbaur Thank you for the suggestions. It's indeed a pain point for some library users who only want to use part of the modules. We will follow up to work on this. Here is the todo card: https://github.com/SAP/contextual-ai/projects/1#card-70787117

Feel free to fork and raise PR if you already have some solutions.