Schema should skip checking attributes stored by DocumentSelection

[scofalik]

As we all know, Schema is not perfect.

There's no use doing big changes or refactoring if we plan to write it from scratch and we haven't really make any decision how it will look.

However, there is one improvement needed right one. If DocumentSelection stores an attribute in an element, that element will not be allowed anywhere, unless we explicitly call Schema.allow().

This is a problem, because a paragraph element with selection:bold attribute is suddenly not allowed in blockquote.

To make it all more managable, the skip would only be implemented in "allow" part of algorithm. That is, you won't have to write things like

schema.allow( { name: '$block', attributes: [ 'selection:bold' ], inside: 'blockquote' } )

but you will be able to do

schema.disallow( { name: '$block', attributes: [ 'selection:bold' ], inside: 'blockquote' } );

It is better and easier to disallow one specific thing than writing all allow rules that would grow bigger with every added feature.