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 c71ae34b2f0cd84635ab216546d3e5e8129b33a8 on ignore-not-supported into 03fa68fa7e16b556e953fe7d52d281e9c1e4ac81 on master.