Display warning on unsupported browsers

[TheJJ]

Mandy uses ES6 and new shit that is only supported in the "latest" browsers (firefox >= 52, ...).
There should be some warning displayed to notify users their browser is too old.

[inakoll]

It should be possible to transpile the es6 code to es5 so that everyone could access Mandy out of the box. I think that is the way es6 is usually handled. I suggest to use webpack and babel to do that.