way to access builtin watcher API

[lifeart]

I found that side-effects in broccoli is quite hard, for example "live-reload" implementation, all we need - is get changed input files, but I can't find simple references to deal with it, all implementations trying to map input to output and it bring computing and FS overhead.

Adding custom watcher also seems not optimal, because broccoli already have builtin (for watched nodes).