core-common: AttributeKey add maybe method

[iRevive]

Resolves #2 (in some way).

Motivation

Adding optional attribute values is rather verbose. To make these operations more bearable, we can allow passing optional values.

[iRevive]

@NthPortal what do you think?

[NthPortal]

I was all set to dismiss this by saying "Option is IterableOnce, so you can just use concat/addAll", but that doesn't solve the case where you have Option[T] rather than Option[Attribute[T]].

on the other hand, should we actually be encouraging the key/value methods? at the very least, I don't think we should be adding additional methods with String key names, but I have a harder time arguing against (AttributeKey[T], Option[T]).

I will have to ponder this some more

[NthPortal]

there's part of me that thinks it would be cleaner to add a single method to AttributeKey to complement apply, such as

trait AttributeKey[T] {
  // ...
  final def maybe(value: Option[T]): Option[Attribute[T]] =
    value.map(apply)
}

[iRevive]

there's part of me that thinks it would be cleaner to add a single method to AttributeKey to complement apply, such as

trait AttributeKey[T] {
  // ...
  final def maybe(value: Option[T]): Option[Attribute[T]] =
    value.map(apply)
}

Brilliant idea. It's much better than the bunch of overloaded methods.