Proposal: FileWatcher

[bugs181]

Example Mockup of what this would consist of. Being able to pass other FileWatch services into this Blueprint would be ideal but not a requirement.

class FileWatcher {
	constructor ( file, data, callback, chokidarOptions, dispose ) {
		const handleWatchEvent = (event) => {
			if ( event === 'rename' || event === 'unlink' ) {
				this.fsWatcher.close();
				dispose();
				callback();
			} else {
				// this is necessary because we get duplicate events...
				const contents = fs.readFileSync( file, 'utf-8' );
				if ( contents !== data ) {
					data = contents;
					callback();
				}
			}
		};

		try {
			if (chokidarOptions) {
				this.fsWatcher = chokidar.watch(file, chokidarOptions).on('all', handleWatchEvent);
			} else {
				this.fsWatcher = fs.watch( file, opts, handleWatchEvent);
			}

			this.fileExists = true;
		} catch ( err ) {
			if ( err.code === 'ENOENT' ) {
				// can't watch files that don't exist (e.g. injected
				// by plugins somehow)
				this.fileExists = false;
			} else {
				throw err;
			}
		}
	}

	close () {
		this.fsWatcher.close();
	}
}