Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
It makes it much easier for people to transition to XHTML5 documents (e.g., EPUB3) or to use XML parsers on the page (like was done in the past for in-page captioning for the multimedia player). The close to XHTML5 we are the more options we have.

Also note the following from [Creating a pull request](http://wet-boew.github.io/wet-boew/docs/pull-en.html):

## Testing your PR before submitting ##

* Validate your HTML markup as to ensure well-formed HTML5.
 * These tests will be run as part of the default build, or by calling grunt htmllint to run the task on your built HTML.
 * You can manually test for well-formed markup using the W3C HTML validator. Validate with an XHTML5 preset and a checkmark next to "Be lax about HTTP Content-Type".