wait improvement and default

[georgir]

Instead of waiting a certain number of seconds, waiting for the last executed command to finish might be desirable.
An option to wait a number of seconds since the command was last ran may also be nice. It differs from the current 'wait interval after file change' in that the first change after some idle interval can be reacted to immediately without delay, and throttling will happen only if changes happen more often than the specified interval.
Also, some kind of wait must be enabled by default, otherwise watch becomes a forkbomb if any kind of rapid file change (such as npm install) occurs in the watched directory.