Recommend Contract.check methods use **kwargs

[seddonym]

It would improve backwards compatibility for Contract.check methods to implement their check methods like so, in case we want to add arguments:

check(graph: ImportGraph, verbose: bool, **kwargs) -> ContractCheck:

Prompted by #138.

[mwgamble]

Perhaps a better approach would be to create a CheckOptions class, and move the verbose parameter in there. Then you can add additional (optional) parameters to that class as desired, and plugin implementers don't have to adjust their code at all just because a new parameter is added.

[seddonym]

Good point. It would result in a breaking change though so I think the change would only be appropriate for a major version.