func (c Chain) Then(j Job) Job {
	for i := range c.wrappers {
		j = c.wrappers[len(c.wrappers)-i-1](j)
	}
	return j
}
Then decorates the given job with all JobWrappers in the chain.

This:
NewChain(m1, m2, m3).Then(job)
is equivalent to:
m1(m2(m3(job)))
package crontab
type JobWrapper func(Job) Job
type Chain struct {
	wrappers []JobWrapper
}
func NewChain(c ...JobWrapper) Chain {
	return Chain{c}
}