[Feature request] Config for only watching some of the events

[meteorlxy]

Currently, we are watching all the events:

watchpack/lib/DirectoryWatcher.js

Lines 67 to 72 in 8d839e8

	this.watcher.on("add", this.onFileAdded.bind(this));
	this.watcher.on("addDir", this.onDirectoryAdded.bind(this));
	this.watcher.on("change", this.onChange.bind(this));
	this.watcher.on("unlink", this.onFileUnlinked.bind(this));
	this.watcher.on("unlinkDir", this.onDirectoryUnlinked.bind(this));
	this.watcher.on("error", this.onWatcherError.bind(this));

In our case (vuejs/vuepress#351), we only want webpack to watch change events, and other events will be handled by ourselves.

[alexander-akait]

@meteorlxy PR welcome 👍

[meteorlxy]

@evilebottnawi

I've created a PR #83 .

Your advice would help a lot.