[Feature request] Configure github to fetch after interval instead of right away

[lookfirst]

Awesome library. Thanks for figuring out all the esoteric unix restart logic! I've learned some good golang tricks from reading your code as well.

The github Fetch() method fetches right away as soon as overseer starts up, but this doesn't work for me because I'm building an agent that goes onto literally thousands of small linux boxes running on a network.

If I start/reboot all the machines at the same time, then I'll DDOS not only github, but also my own network if there is an upgrade to be had.

All I'm asking for is the ability to configure this behavior. Ideally, just fetch after the first interval (which I randomize during configure time to prevent the DDOS).

As a short term solution, I've just hacked my own fetcher from your code, but that isn't obviously the right long term solution.