Checklist for this pull request

• Your code should contain tests relevant for the problem you are solving.
• Test is included
• Your commits messages format should follow the jQuery git commit message format (http://contribute.jquery.org/commits-and-pull-requests/#commit-guidelines).
• Same as title
• The pull request should reference existing issues or link to a reproducible demo.
• Referenced issue (feature request) isValidate Web Components #2492

Description

This PR tries to add validation support for custom elements (Web Components). See#2492 for a first technical analysis.

It's possible that the current implementation is not good enough. Especially the part where I added the customElements to both, the "focusin.validate focusout.validate keyup.validate" events and to the "click.validate" events. Maybe we would need to try to differ there or we should go with "more settings value" mentioned in#2492 approach?