Implements right prompt support

Description

Added support for right prompt and updated documentation.

Motivation and Context

Original Spaceship theme provide it. I tried fish as a replacement to ZSH and I was really missing this thing. I didn't find issues on this thing but it's implementation seemed to be easy so I just did it.

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Screenshots (if appropriate):

How Has This Been Tested?

Ran it on my own machine. ArchLinux, Alacritty / Termite terminals, fish version 2.7.1.

[ ] I have tested using MacOS
[x] I have tested using Linux

Checklist:

[x] I have checked that no other PR duplicates mine
[x] My code follows the code style of this project.
[x] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[ ] I have updated the tests accordingly. NOT REQUIRED

matchai / spacefish

Implements right prompt support #101