Implements fetcher runner by Cron instead of Config.MinFetchInterval

[moisespsena]

Using default cron instance

import (
	"github.com/robfig/cron"
)

func main() {
	schedule, _ := cron.Parse("@daily")

	overseer.Run(overseer.Config{
		Program: prog,
		Address: ":3000",
		
		FetchCronSchedule: &schedule,
	})
}

Using your cron instance

import (
	"github.com/robfig/cron"
)

func main() {
	Cron := cron.New()
	Cron.Start()
	defer Cron.Stop()
	
	schedule, _ := cron.Parse("@daily")

	overseer.Run(overseer.Config{
		Program: prog,
		Address: ":3000",
		
		Cron: Cron,
		FetchCronSchedule: &schedule,
	})
}

[jpillora]

Thanks for the PR, however I intentionally left out the fetch interval from the main config, and left it to each fetcher to implement their own simple logic. For example the S3 fetcher, defaults to 5 minutes whereas the file fetcher defaults to 1 second, if the setting were in the main config (in the form of a duration or a cron schedule) then sane defaults would not be possible. I should probably make this clearer in the docs.

Maybe a cron helper might be useful:

func main() {
	overseer.Run(overseer.Config{
		Program: prog,
		Address: ":3000",
		Fetcher: &fetcher.HTTP{
			URL:      "http://localhost:4000/binaries/myapp",
			Interval: overseer.Cron("@daily"),
		},
	})
}

It would not need to return an error as this the user would not be writing cron strings, but rather it be hard-coded, or it be a selection of 4 interval options.