Make Buble an optional dependency

Some people prefer to use Babel over Buble which can cause issues as Buble is a dependency thus remains installed (otherwise laravel-elixir-webpack-official will complain). Other packages such as vue-loader priorities Buble over Babel when it comes to .vue files and that's not suitable in many cases seeing as Buble lacks support for things that Babel has support for.

I've also suggested vue-loader to have an option of sorts to use Babel over Buble in the case that both are installed. There is no doubt that there are other libraries that may contain a similar issue.

I understand that to remove Buble as a dependency all together could be a breaking change as there will be some people that rely on Buble, and changing to Babel isn't a good idea either as some people may prefer Buble.

What I think might be possible is to make Buble an optional dependency, and to only use it if it is installed. I don't think that'll become a breaking change as making it optional will not cause it to be uninstalled so it will still be used.