Added optional parameter withing

[bboyle1234]

#102

[amis92]

I'm glad you built this so quickly and easily, working simple code is the best code.

However:

• Any new Feature should be first hidden behind a new Features flag.
• Attributes namespace/package isn't the best place for embedding Optional. Also embedding is the last solution I'd like to go with. Let's keep discussing this in A different kind of Wither: With(Optional...) #102.
• Making the Optional parameters in doesn't really help performance, I think. It's a small struct.