Add extended data validation library support for method parameters

[zmingee]

There's currently some basic data validation for methods, but it only supports type validation (i.e., verify parameter is dict, int, str, etc.). It would be nice if users could utilize an external library like Cerberus to validate the data inside the extension rather than unpacking all of the variables inside of the view itself.

I could probably get the feature implemented myself and submit a pull request, but as I said in the last issue report I opened, there aren't any contributing guidelines so I'm not sure what the next step of that would be.

[nycholas]

Hello,

Very cool this project that would help a lot. Would be some changes that break old code, then I will look for the next version.

Thank you.