Add OnMissing callback

[adregner]

Not sure if this is of interest for many others or the project in general, but it was useful for me and thought I'd share.

My use case is pretty simple and I wanted to keep it simple by adding this so I can use it as a self-filling cache. The many different ways that Cache looks for values is duplicated all over so I didn't bother refactoring that all to use this callback unless this idea is of use to others in which case I can do that too. I presume there are performance implications to doing that though.