The init can select the selector element in a context dom node passed by a parameter

[livike]

First of all thanks for the nice emoji picker. It's just what I needed.
Hovewer i neede a little extension and for this i created this PR.
In my project i need to construct a DOM node before the document object is created. In this case the wdtEmojiBundle.init did not find the element I wanted to add the picker.
I extended the function with a new parameter that accepts the context dom node.
For standard use, when the context is a document object, the parameter is not passed and the context is set to a document.

I had some issues with a gulp build, so mazbe this PR will be not accettable. Can you please provide some instructions how you want gulp to be runned?