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 
I understand your reasoning, but I don't like having this defined in multiple places. There should be one authoritative source and currently, that's the database. Otherwise we'll have to remember all the places we have to modify code when we want to add a new plugin type.