Expose current HystrixCommand to fns

@ohpauleez noticed that the current HystrixCommand was inaccessible from
:fallback-fn. This change introduces a dynamic var, *command* that
exposes the currently running HystrixCommand instance so it can be
interrogated as needed. At some point, if the dynamic var doesn't work
out or proves error-prone or unwieldy, we may introduce alternative
functions that explicitly pass the function as the first arg.