Added fish config and made config as modular as possible

[DanielMSchmidt]

Hi there,

I just had time to add the fish shell. Here is what I did and what (IMHO) needs to be done now or in future iterations:

• [X] fish, z, nova
• [X] Ability to change the fish theme
• [X] Ability to add other themes
• [ ] Support for non-OSX systems
• [ ] Documentation regarding the setup of the whole ecosystem
• [ ] A good system for default variables / required variables
• [ ] Support for other shells

Hope we can discuss the best next steps within this PR. Regards and happy holidays!

[DanielMSchmidt]

Will do a new big PR this or next week with all my changes, hope you will find the time to review that. Sorry for not updating this PR :(