{after,before}_command hooks should be unified with Pry::Hooks system

`Pry.commands.after_command("edit")` becomes `Pry.hooks.add_hook(:after_edit, :name)`

`Pry.commands.after_command("!!!")` becomes `Pry.hooks.add_hook(:"after_!!!", :name)` 

Note the symbol 'string' in the last example as `:after_!!!` is not a valid symbol

Consider making hook names 'indifferent' to string/symbols, so `"after_!!!"` is the same as `:"after_!!!"`
