Add the option to ignore starting some of the timers

[rbarry82]

In case only some of the interval timers are desired, add an option to conditionally ignore starting some of them.

Closes #27

[rbarry82]

@csmarchbanks are you still maintaining this?

[csmarchbanks]

Hi @rbarry82, I apologize for the slow response. I have had very little time to maintain this repo anymore, and there is a discussion going on to transfer it to prometheus-community: https://github.com/prometheus-community/community/issues/39, depending on how that goes I will try to find some time to review this soon.