[osis-core] level attrib added to <q>

[Troy A. Griffitts]

<q> should contain an optional 'level' attribute to allow designation of 
the depth of the quote, e.g.

John said, <q level="1">The people say, <q level="2">Where is our 
God?</q>.  But I say: open your eyes!</q>


All quotes at certain level can be rendered with the same ", ', `, or 
what-have-you, characters.

(I realize this can possibly be derived, but this is so common, that it 
is much easier to tag quotes inside quotes with level="2", than to walk 
the parent stack for each quote to see if it is a subquote.)