Variable mapping: Allow In(:strict)

[apotonick]

When using In() => {:model => :internal_name} there's no implicit check if ctx[:model] was actually set for the incoming ctx.

Maybe a strict mode like

In(strict: true) => {:model => :internal_name}

would make :model a required keyword in the automatically created filter proc (although I don't know how to do this without meta-programming :brrrr:).

[apotonick]

The same for Out()! 🌞