Added event hook `filterPlaintext` to intercept a BBcode element's content

Allows you to edit only the elements content while leaving untouched all tags.

For the next major version don't forget to add `filterPlaintext` method to the `Decoda\Hook` interface.
Fixed emotions detection inside of a bbcode element

Execute emoticon detection on `filterPlaintext` event instead of `beforeParse`.

There is a limitation when a tag does not exists on the parser, therefore an emoticon glued to it can not be detected.

Performances are probably reduced because it execute a regex for each elements content.