Overview of changes:

 *  cached() will not always create cache

    Turns out caching *any* result is not the best approach as it means
    failures will keep manifesting in cache.  New version solves this by
    caching only if exit status is zero.  This can be overrided per
    call.

If exit status is not zero, do not create cache, but allow user to
override the default match.

Overview of changes:

 *  First implementation