Improved Markdown.set_output_format()

Specificaly, `self.output_format` is defined and contains a string of the
output format used on the instance. This is more useful that an instance
of the searializer when determining alternate behavior elsewhere in the parser.
For example, see Issue #129.

Also cleaned up the error when an invalid format is provided. We now re-raise
the original error (with a custom message) rather than raising a new error.

markdown/__init__.py

@@ -238,11 +238,17 @@ def reset(self):
 
     def set_output_format(self, format):
         """ Set the output format for the class instance. """
+        self.output_format = format.lower()
         try:
-            self.serializer = self.output_formats[format.lower()]
-        except KeyError:
-            raise KeyError('Invalid Output Format: "%s". Use one of %s.' \
-                               % (format, self.output_formats.keys()))
+            self.serializer = self.output_formats[self.output_format]
+        except KeyError, e:
+            valid_formats = self.output_formats.keys()
+            valid_formats.sort()
+            message = 'Invalid Output Format: "%s". Use one of %s.' \
+                       % (self.output_format, 
+                          '"' + '", "'.join(valid_formats) + '"')
+            e.args = (message,) + e.args[1:]
+            raise
         return self
 
     def convert(self, source):