diff --git a/IPython/nbconvert/filters/strings.py b/IPython/nbconvert/filters/strings.py
index 5464e7d..aa51fcc 100755
--- a/IPython/nbconvert/filters/strings.py
+++ b/IPython/nbconvert/filters/strings.py
@@ -49,6 +49,13 @@ def wrap(text, width=100):
     wrpd = map('\n'.join, wrp)
     return '\n'.join(wrpd)
 
+def single_line(text):
+    """Wrap multi-line text into a single line
+    
+    Used in markdown heading cells, which are not allowed to be multiline.
+    """
+    return ''.join(text.splitlines())
+
 
 def strip_dollars(text):
     """
diff --git a/IPython/nbconvert/utils/pandoc.py b/IPython/nbconvert/utils/pandoc.py
index c914cee..8612970 100644
--- a/IPython/nbconvert/utils/pandoc.py
+++ b/IPython/nbconvert/utils/pandoc.py
@@ -24,7 +24,7 @@ from IPython.utils.py3compat import cast_bytes
 # Classes and functions
 #-----------------------------------------------------------------------------
 
-def pandoc(source, fmt, to, encoding='utf-8'):
+def pandoc(source, fmt, to, extra_args=None, encoding='utf-8'):
     """Convert an input string in format `from` to format `to` via pandoc.
 
     This function will raise an error if pandoc is not installed.
@@ -44,7 +44,10 @@ def pandoc(source, fmt, to, encoding='utf-8'):
     out : unicode
       Output as returned by pandoc.
     """
-    p = subprocess.Popen(['pandoc', '-f', fmt, '-t', to],
+    command = ['pandoc', '-f', fmt, '-t', to]
+    if extra_args:
+        command = command + extra_args
+    p = subprocess.Popen(command,
                          stdin=subprocess.PIPE, stdout=subprocess.PIPE
     )
     out, _ = p.communicate(cast_bytes(source, encoding))