Allow additional optional and rest arguments in classes that implement interfaces

[Verseth]

Hi, I noticed that method signature need to match exactly when using interfaces.
I found it quite annoying since some libraries define getter methods with additional unused rest arguments eg. ActiveRecord::Attributes. This means that I would have to define the interface with additional rest arguments and also add those rest arguments to regular manual getters in other classes.

This PR allows classes that implements interfaces to define additional optional or rest arguments.

module InterfaceOne
  def foo(a, b); end
  def bar(a); end
end

class KlassOne
  extend Interfaceable
  implements InterfaceOne

  # this will be accepted
  def foo(a, b, c = 4); end

  # this will be accepted as well
  def bar(a, *args, **kwargs); end
end

module InterfaceTwo
  def foo(a, b, c = 5); end
  def bar(a, *args); end
end

class KlassTwo
  extend Interfaceable
  implements InterfaceTwo

  # this will not be accepted
  def foo(a, b); end

  # this is invalid
  def bar(a); end
end

[artemave]

This looks good to me. The only thing, I would add new tests (where applicable) rather than adding to the existing ones.

[artemave]

Oh, and please also update the readme.

[Verseth]

Sure, I've just added new test cases and updated the readme

[artemave]

Thank you for your pr. I pushed an updated version of the gem.