Allow for sending in classes for all individual elements (especially required when using tailwind)

[emattias]

When styling with tailwind you send in classes for everything you style, so it would be nice to have args for that, like: @beforeOptionsInputClass, @optionClass, etc.

[mayatron]

Something like this would be super useful for me as well. It would be really helpful in some cases to be able to more easily style parts of Ember Power Select without the overhead of providing fully custom components.

If we wanted to add corresponding @...Class arguments support to each of the composable components, we would add the following, as additional classes to the default ones included by Ember Power Select.

• @afterOptionsClass
• @beforeOptionsClass
• @groupClass
• @optionsClass
• @optionClass (not a composable component, but useful)
• @searchMessageClass
• @selectedItemClass

@cibernox thoughts on this? I'd be happy to draft a PR for this (including doc changes and test coverage), if this is something you're open to considering. Happy to discuss any feedback you have.