Fix testing on Windows machines.

Windows carriage return line endings were not being tested for.

Fixes #2087.

test/test-markdown-external.html

@@ -18,7 +18,7 @@
 		<div class="reveal" style="display: none;">
 
 			<div class="slides">
-				<section data-markdown="simple.md" data-separator="^\n\n\n" data-separator-vertical="^\n\n"></section>
+				<section data-markdown="simple.md" data-separator="^\r?\n\r?\n\r?\n" data-separator-vertical="^\r?\n\r?\n"></section>
 			</div>
 
 		</div>