Config to enable/disable minification

Package config that will allow minification to be turned on/off (see #6). It works but I just need to update the tests and docs accordingly.

- [x] Publish package config file

```bash
$ php artisan config:publish fitztrev/laravel-html-minify
```
- [x] Read config setting and determine whether or not to minify
- [x] Update tests to check output when `enabled` is **true**
- [x] Update tests to check output when `enabled` is **false**
- [x] Update README with instructions for `config:publish`