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

The functions themselves no longer do that.

Leave that up to the caller.

Leave that up to the caller.

Layout and links

Per conversation with @bpinto in Gitter.

There's no need for two separate directories. You don't have a `.git` and `.git-custom` folder, you just put your config in `.git` :)

The most straightforward interpretation of XDG basedir spec is that user configuration for omf would go in `~/.config/omf`, so let's put it there. The only question is whether omf-generated config (i.e. the `theme` file) should go there as well. By analogy with git, programmatically generated config should probably be merged in with user config. This also makes it so when a user clones their dotfiles to a new machine, both kinds of settings come with it.

Add local-config

Rename 'get' to 'install'

No def plus template

We already use 'install' on the command description: 'Install one or
more packages.'

Also call update using 'u' instead of 'p'.