Class methods

This allows missingly to define class methods as well for example:

```ruby
class Thing
    include Missingly::Matchers
    
    handle_missingly /^find_by_(\w+)$/, class_method: true do |matches, *args, &block|
        SomeApi.search(field: matches[1], value: args[0])
    end
end

Thing.find_by_name("foo")
```

To make a handle_missingly call define class methods just set the option `class_method` to true.

This means that we can use this to provide ActiveRecord like system around our classes.

I've tested all the matchers when used as class methods and it all works.