Add func (c *FormatCheckerChain) List() []string

[fenollp]

My usage is to be able to precisely control which format checkers are in the chain.

My current workaround is to reset the chain with

func init() {
	gojsonschema.FormatCheckers = gojsonschema.FormatCheckerChain{}
	gojsonschema.FormatCheckers.Add(...)
	...
}

still, this may help someone else somehow :)

[Surfoo]

Hello,
This repository is not maintained anymore, the new one is https://github.com/gojsonschema/gojsonschema, could you submit your PR on this one? Thank you!