Issue title: Disable manifest file generation
 Issue body: Is there a way to disable generation of the `mix-manifest.json` file?
 Comments: 
 Comment 0: No. If you don't need it, add a post compile script that does something like `rm public/mix-manifest.json`. 
 Comment 1: Thank you.
 Comment 2: Well, this command is not cross-platform...
{'source': 'GithubIssues', 'repo_name_with_owner': 'JeffreyWay/laravel-mix', 'issue_id': 386856034, 'repo_stars': 4975.0}