Add support for interval offsets

[WilsonGramer]

Adds the ability to offset a Task by a specific interval from its Plan, such so that the Task calculates its next run based on adding said interval to the Plan's next interval. This can be useful if one has a preexisting Plan and needs to uniformly adjust the times at which its Task runs (eg. timezone differences, etc.).

Usage examples:

plan.do(offsetBy: 3.hours) { ... }

let calculateOffset = { () -> Interval? in
  // some computation to calculate offsets on the fly
}
plan.do(offsetBy: calculateOffset) { ... }

[codecov-io]

Codecov Report

Merging #28 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   90.86%   90.96%   +0.09%     
==========================================
  Files          16       16              
  Lines         832      841       +9     
==========================================
+ Hits          756      765       +9     
  Misses         76       76

[luoxiu]

Hi @Wilsonator5000 , Sorry for my late reply! I like the offset idea! But i think it should be part of the plan, not the task, so this may be better:

extension Plan {
    // just a draft
    func offset(_ body: @autoclosure @escaping () -> Interval?) -> Plan {
        return Plan.make { () -> AnyIterator<Interval> in
            let it = self.makeIterator()
            return AnyIterator {
                if let next = it.next() {
                    return next + (body() ?? 0.second)
                }
                return nil
            }
        }
    }
}

And since it's actually a transformation, a map operator seems better, what do you think?

[WilsonGramer]

Hi @jianstm, your method makes sense too! A map wouldn't work because it would convert the iterator to an array, and we want the the intervalOffset body to be evaluated when the next run time is calculated (which could be far in the future). I'll look into the iterator method, though.