Add DeciderBuilder that can be used to build deciders.

This PR adds `DeciderBuilder` that can build Decider functions.
Its implemented so the user can use collection initializer to create it, and it  
can be implicitly casted to a decider function so it can be used directly when creating a `SupervisorStrategy`
```C#
var strategy = new OneForOneStrategy(new DeciderBuilder {
   { typeof(SomeException), Directive.Restart },
   { typeof(SomeOtherException), Directive.Stop },
   { typeof(SomeThirdException), e => e.SomeValue ? Directive.Restart : Directive.Escalate },
}));
```
By default it fallbacks to DefaultStrategy, but other fallbacks can be supplied in the constructor.

Docs and Tests included.