Introduce the CKEDITOR.isDebug property

[Reinmar]

Related to https://github.com/ckeditor/ckeditor5-core/issues/38.

We need a property which will allow us to conditionally disable/enable some debug logs. For instance it could be used as follows:

if ( CKEDITOR.isDebug ) {
    if ( doSomeSuperUnnecessaryDebugChecks() ) {
        throw new CKEditorError( 'sth-broke: Kaboom!' );
    }
}

Later on we'll need to work on #27 to make it fully functional.

The idea here is that this isn't necessarily constant (as it might need to be switchable while debugging something) and that it's something different than already existing CKEDITOR.isDev (as we may want to build a debug version of CKEditor).