Add self update command

[matt-allan]

See https://laravel-zero.com/docs/build-a-standalone-application/

[matt-allan]

This is going to be kind of complicated.

The default Github strategy uses releases which is what we want. However Laravel Zero overrides it to assume the phar is stored with the code.

The overridden class is registered in a 'component provider' which is a Laravel Zero concept and it isn't clear how to override those bindings.