HTML Validation: Ability to ignore custom htm tags (e.g. <ion-*>)

New Preference Page and Quick Fix were added in order to make it possible to use the new WTP 3.6.3 feature on
controlling the element names validation. The feature allows to skip from validation some HTML5 tags.
JUnit Test case is added to test Quick Fix appearance on Unknown Tag error for custom html tags

Signed-off-by: vrubezhny <vrubezhny@exadel.com>