Add emacs plugin

Incorporate changes after code review of #408

Updated in response to comments from @bucaran

Port of oh-my-zsh emacs plugin.
Includes a function to kill emacs from command line once
a daemon has started.

More information included in plugins/emacs/README.md.

See reference plugin here:
https://github.com/robbyrussell/oh-my-zsh/tree/master/plugins/emacs

Awesome README + @marcker's Logo

fix link to plugin repo

ta-to-done 📒

Added a set fish_theme_l_right_prompt true variable to activate the right_prompt for l

Characteristics

- Working directory
- Git branch and dirty state (if inside a git repo)
- Rbenv Ruby Version
- Rbenv Ruby Gemset

use existent plugins as examples in the README.md

closing #397

Fixes a small bug

Fixes https://github.com/fish-shell/fish-shell/issues/1964

Add 'will' theme. (double turnstile ⊨)

Fix for better-alias plugin

Improve Markdown indentation

Fix Markdown list indentation