Strip entire inline element after removing required attr/style

[Piotrek Koszuliński]

E.g. <a name=""></a> should be removed when name attribute was filtered out.

Similar case: currently <span> is allowed because plugins like font, colorbutton allow styles which base on span elements. So when pasting e.g. <span lang="pl">, lang is filtered out, but <span> left.

In fact, span is a special case, because when it doesn't have any attrs/styles/classes it should be removed automatically. So, to simplify implementation we can make two fixes:

1. There should be a way to mark attrs/style/class as required. Element is marked by this rule as valid (element has to be valid, otherwise it will be filtered out) only if it has this required property.
2. After applying filter, spans without attributes should be removed.