Allow to configure grace period

[king-of-poppk]

The grace period for token validation is currently hard-coded to 60 seconds, see here:

jose/R/jwt.R

Lines 171 to 188 in 429a463

	# As suggested in the spec, we give a 60s grace period to account
	# for inaccurate clocks.
	check_expiration_time <- function(payload){
	if(length(payload$exp)){
	stopifnot("exp claim is a number" = is.numeric(payload$exp))
	expdate <- structure(payload$exp, class = c("POSIXct", "POSIXt"))
	if(expdate < (Sys.time() - 60)){
	stop(paste("Token has expired on", expdate), call. = FALSE)
	}
	}
	if(length(payload$nbf)){
	stopifnot("nbf claim is a number" = is.numeric(payload$nbf))
	nbfdate <- structure(payload$nbf, class = c("POSIXct", "POSIXt"))
	if(nbfdate > (Sys.time() + 60)){
	stop(paste("Token is not valid before", nbfdate), call. = FALSE)
	}
	}
	}

Would it be possible to make this configurable? It would help to test the token expiration functionality.