Add Laravel Mix HMR feature compatibility

[LeoColomb]

This little addition targets to provide support for Laravel Mix "native" Hot Module Replacement for all Acorn-based development.

"Native" means BrowserSync is no more required, neither any proxying tools.

• User documentation: https://laravel-mix.com/docs/5.0/hot-module-replacement
• Laravel source: https://github.com/laravel/framework/blob/7ebd21193df520d78269d7abd740537a2fae889e/src/Illuminate/Foundation/Mix.php#L32-L40

Pros

• Use all the power of webpack-dev-server via laravel-mix
• Usage is really straightforward
  • No special/additional configuration
  • No additional dependencies
• No more proxy required
  • The same address can be used with or without HMR
  • HMR handling script is added to build assets automatically
• We can get rid of browsersync
  • BS is really hard to configure, specially with a PHP development (just take a look how many issue/pr are related to BS)
  • BS is not actively maintained anymore
• The addition won't change anything if laraval-mix HMR is not used
• The trait can be reused by any assets manifest class

Cons

• By using this, we loose browsersync special features, like multi-tab management. (did we really use them? 😇)