Restore proper pattern for attributes in elements.

It is important that test/test-element-attributes-markdown.html do test
the ability for the user to define its own pattern.

test/test-element-attributes-markdown.html

@@ -22,7 +22,7 @@
 				<!-- <section data-markdown="example.md" data-separator="^\n\n\n" data-vertical="^\n\n"></section> -->
 
 				<!-- Slides are separated by newline + three dashes + newline, vertical slides identical but two dashes -->
-				<section data-markdown data-separator="^\n---\n$" data-vertical="^\n--\n$">
+				<section data-markdown data-separator="^\n---\n$" data-vertical="^\n--\n$" data-element-attributes="{_\s*?([^}]+?)}">>
 					<script type="text/template">
 						## Slide 1.1 {_class="fragment fade-out" data-fragment-index="1"}