Remove the jQuery dependency

[gael-ian]

A Vanilla JS version has been a long time feature request on the original Cocoon but, so far, only alternative reimplementation by other developers have been published to npm:

• https://github.com/dabroz/cocoon-js-vanilla
• https://github.com/thedanbob/cocoon/blob/master/app/assets/javascripts/cocoon.es6.js
• https://github.com/oddcamp/cocoon-vanilla-js

None of them support the additions made in Cocooned.

As jQuery is still in use in many projects, we should keep a compatibility layer with it.

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Originally requested in nathanvda/cocoon#580, nathanvda/cocoon#537, proposed in nathanvda/cocoon#579

[gael-ian]

Another step in this direction has been made with #18. Cocooned does not use jQuery itself but we still need a compatibility layer to ease transition and to accommodate applications where jQuery is still welcome.

[gael-ian]

Fixed in #22