Improve OMF CLI completions

This is in response to an issue we had on the original repository.
It's been some time since I did the actual work, so a double check would be nice.

+ Completions for `remove`
+ `cd` and `install` show both plugins and themes
+ `theme` shows only installed themes

See merge request !1

docs: update Themes.md

Add Visual Studio Code plugin

Command line for Visual Studio Code

update to oh-my-fish repo

Added a new theme `es` with properly formatted remote name 

Remove themes doc travis test

git_ahead: Fix indicator override

Add chain theme to db

Fixes #186 by removing the PATH save/restore code from init.fish.

There isn't a single occurence where Oh My Fish code changes the
value of PATH variable.

Plugin code which changes the PATH variable should be aware of
the side-effects and manage PATHs correctly, avoiding duplication.

Tiny text improvements

some grammar/punctuation mistakes

bugfix: git_ahead no longer reports 'none' as 'ahead'

git_ahead: Rewrite code, fix ahead being reported as behind

registry: Add lolfish theme

registry: add hasanozgan/theme-lambda

Update theme docs (Themes.md)

registry: add rafaelrinaldi/theme-pure

This updates omf plugin to new architecture, moving
functions to `functions` directory. It also revamps the
separation of concerns between CLI and underlying
implementations, organizing them into directories,
whose are autoloaded in plugin startup.

Remove update instructions

Oh My Fish has been stable for many months, we no longer need the update
instructions on the main README.

Update package: fasd

Note for fasd:

Existing fasd plugin doesn't even work properly.

- No fasd_cd
- No default aliases (f, a, s, d, z, zz, ...)

So I forked it and totally rewrote it. Please consider it as different
plugin.

New theme: shellder