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.

Proposed fix:

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.

Possible format:

"a[!href]; span{!color}"

PS. Plugins like font and colorbutton use styles to register content they generate, so by default when passing style to filter#allow() all parts of style should be marked as required.