parseOptions: allow a way to keep errors silent (exception only)

[nfischer]

Currently, parseOptions always outputs an error message to stderr if an unknown option is encountered. It would be great if there was an easy way to make sure parseOptions would remain silent, even when it throws an error.

My suggestion is to change the function signature to

function parseOptions(optString, optionsMapping, functionOptions)

The last argument could have a silent option which, if specified, should take precedence over config.silent inside common.error().

Blocks #590