DTD caching makes it impossible to modify DTD even before first editor creation

[Piotrek Koszuliński]

CKEDITOR.dtd.customtag = { em:1 };
CKEDITOR.dtd.$block.customtag = 1;
CKEDITOR.dtd.body.customtag = 1;

Load:

<customtag>foo</customtag>

It will be transformed to:

<customtag></customtag>

<p>foo</p>

I think that we don't have to rewrite all code which caches DTD objects because that would ruin performance or increase complexity in some places. Therefore we can introduce event CKEDITOR#dtd fired when DTD is created and allowing its modifications.

Based on: ​http://stackoverflow.com/questions/16066556/ckeditor-how-to-allow-for-inserthtmlcustomtag-myattr-value-customtag