Instant plugins reload

Changes include:
* all plugin reload logic gathered in one `loadOrConfigPlugin` fn,
* each plugin can now perform specific tasks just after its load `onChangeCb`,
* code DRY'ed,
* debug logs now show ticker source.

Closes #38 
Yeah, you were right, there's no need to use ```apply``` here. Just add a comment like "git-rev functions need to be wrapped because they don't return err as first argument in callback".