Common mechanism for validating backend parameters

[juhoinkinen]

Now some backends validate the type of their parameters (at least vw_multi), and also the value of the common input_limit parameter is validated.

There could be a common mechanism for validating both the correct type and value of backend parameters, at least of the parameters that are common to all backends (limit, input_limit, and the ones possibly added in the future, e.g. a threshold for language detection probability).

[osma]

Agreed!