added parameters to validation

[ae0000]

I added the ability to provide parameters to the validation, like: maxLength[123]

Unfortunately it breaks the API by requiring the validation functions to accept an extra parameter.

But - thought I'd sent it back in case you wanted it :)

[mccoyst]

This is cool, and thank you very much for the PR, but I don't like breaking the API. Maybe separate parameterized validators into their own type with its own Validate method? E.g. type P map[string]func(interface{}, []string) error. The existing V could ignore validators with params, and P could ignore those without. Something like that — I'll have to put more thought into it.

This also needs a bit more robust checking of the syntax of parameters. It at least needs to verify that the closing ] is present. And if we want to go the extra mile, allow | to be escaped so that it can appear in params.