Added right prompt support (fish and zsh)

[thecaralice]

Closes #23.
Changed $SILVER to $SILVER_LEFT, $SILVER_SEPARATOR to $SILVER_LEFT_SEPARATOR etc., also silver print is now silver lprint
For bash, zsh and fish edited init files for backwards compatibility (needs also powershell).
Also prompts can be empty now

TODO:

• Edit powershell script to get backwards support
  • Check Windows support
• Refactor to remove code duplicated in print functions
• Update readme.md
  • Change variables to the new ones
  • Include information about right prompt
• Update wiki
• Update Cargo.toml

[thecaralice]

Right prompt is working now, needs documentation updates. GitHub doesn't have support for wiki PR's so I con't edit it.

[thecaralice]

Note: Powershell script only works on Powershell Core 7+

[thecaralice]

Closing to change the source branch