Variable mapping: Allow In(:strict)

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:).