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 
In this case callback params are the problem, not sure how `async.apply` can help here.

`git-rev` callback skips `err`, and just calls fn with a value.