Add restart-on-crash functionality

[yarreg]

Something like "-r" options in forego

[ilgooz]

it could be useful restarting died jobs due to an error(exit code >=1). only jobs started with -r option should have this restart policy. e.g. syntax: spm start -r job1 job2 or spm start -r. -r option should accept a go duration value(default=0) which is the wait time before doing a restart on died jobs. e.g. syntax spm start -r 10s job1 job2.

https://golang.org/pkg/time/#ParseDuration

cc @ufukomer