a context object with circular references will explode the stack on parse

[ceejbot]

I'm attempting to use the new headless parsing feature to write a Slack bot, following the example in the pirate joe repo. If I pass a context object to the parse() command that happens to include circular references (as many node http request clients or messages do), yargs will never succeed in parsing it. Eventually the process will exceed the maximum call stack size and crash.

In my particular case, I made an RTM client using the @slack/client module and hung it onto the context object. That should suffice to repro.

[nexdrew]

Closing since this was fixed by #675. Thanks for reporting!