Use a MULTILINE anchored regex when removing carriage returns

This seems a lot faster on long strings (8s down to 8ms for a 120000
character string).

Author: charmasaur

nbval/plugin.py

@@ -795,7 +795,7 @@ def sanitize(self, s):
         return s
 
 
-carriagereturn_pat = re.compile(r'.*\r(?=[^\n])')
+carriagereturn_pat = re.compile(r'^.*\r(?=[^\n])', re.MULTILINE)
 backspace_pat = re.compile(r'[^\n]\b')