text: Title: Parameters to provide a `validate` method
  Body: Hi there!
  
  I'm thinking of adding a 'validate' method to the Parameter class in the OpenAPI module.  This would check the given value (e.g. in a query) actually matched the type (and format, if set) of the parameter.
  
  A further possibility would be to provide a `from_request` method which searched for the value in the request structure according to where it would be found (header, query, path, etc), and returned the value - or the parameter's default if the parameter was not found in the request.
  
  What do you think?  Do you want me to write that?
  
  Have fun,
  
  Paul