implements help for commands

Fix #545: Remove custom packages and themes

Remove function was not deleting packages and themes from the OMF_CONFIG
folder.

Update repo for local-config plugin

Since they're plugins again :P

describe command

to easily see from commandline what each package does via their descriptions
on github

registry: add foreign-env plugin

Update rbenv plugin git location

registry: add title plugin

Reference #517 Add default theme to database

cd command

Jump around Oh My Fish packages and themes using `omf cd <name>`. If no
<name> is specified, jump to oh my fish root folder.

Instead of clobbering the actions with both version and help, use
options instead. They are also going to be used for subcommands (e.g.
omf list --help) once they support options.

Change theme when creating a new theme

We already change the current directory to the theme dir, to make it
easier for development, we also change the current theme.

Extracting template to a file and improving it

It's hard to read the template as it was being created using `echo`
instead of being defined on a separate file. Also adding some comments
to improve the configuration file readability.

Improve README clarity for custom configuration

The second argument (background) was added on fish 2.2

Fixes #524.

Fish should set default colors:
https://github.com/fish-shell/fish-shell/blob/b32f770a69527eca4e76b9e8f648a39da17911cb/share/functions/__fish_config_interactive.fish

But in case some users don't have these color variables set, use their
defaults values.

Having a clear namespace `omf.` improves the readability of the code as
we clarify what is the function name and what is the namespace.

Instead of using custom colors, reuse fish's colors.
Original fish colors here:
https://github.com/fish-shell/oh-my-fish/blob/4d628d5f43dbe8b138605d1f6ecbd762187afd7f/functions/restore_original_fish_colors.fish