Babel: Disable wrapping

* lisp/ob-core.el (org-babel-insert-result): If the argument to ':wrap'
is 'nowrap', don't wrap the results.

* doc/org.texi (Header Arguments): Describe 'nowrap' behaviour.

This allows the results of a block to be inserted without wrapping, by
overriding a ':wrap' property defined in an enclosing scope.