Add ParseOption/DeviateOption to ignore not-supported statements

[wenovus]

// DeviateOptions contains options for how deviations are handled.
type DeviateOptions struct {
	// IgnoreDeviateNotSupported indicates to the parser to retain nodes
	// that are marked with "deviate not-supported". An example use case is
	// where the user wants to interact with different targets that have
	// different support for a leaf without having to use a second instance
	// of an AST.
	IgnoreDeviateNotSupported bool
}

[coveralls]

Coverage decreased (-0.02%) to 84.006% when pulling c71ae34 on ignore-not-supported into 03fa68f on master.