Current in use theme should not be deletable

`default` was doing nothing.

Rename `omf use` to `omf theme`.

The first time I saw `use`, I thought I could use the command for
everything, including packages, but it's only used for themes. So, it's
better to have a clearer name.

United States of the Fish → Wahoo → Oh My Fish!

Only show deprecation warnings when omf is loaded.

Fix error handling when github API is unavailable.

The `exit` (instead of `return`) caused the shell to quit if there was
an error with github.

README: add title and foreign-env plugins

The oh-my-fish.fish startup file is sourced when changing themes after
the `fish_theme` variable is set.  The deprecation warning should only
be shown if the user sets the deprecated variables.

Add omf commands for listing available plugins/themes and for downloading a plugin/theme

fixes #496 Do not throw warnings when theme is not set

Fixed typo in CONTRIBUTING.md