Post-processor to check specific types

[gdaniel]

This could complement the checks already performed by the intent recognition provider.

For example, DialogFlow has some type checking for emails, but still maps obviously wrong emails. On the other hand the RegEx matcher does not provide any check for email.

This could be handled in a generic way in a post-processor.

[gdaniel]

Related to #46: if we are able to discard context parameters we definitely need a mechanism to ask them again