bake: Big pass on bake performance. - Reduce the amount of data passed between processes. - Make inter-process data simple objects to make it easier to test with alternatives to pickle. - Make sources have the basic requirement to be able to find a content item from an item spec (path). - Make Hoedown the default Markdown formatter.

--- a/piecrust/sources/interfaces.py	Fri Nov 03 23:14:56 2017 -0700
+++ b/piecrust/sources/interfaces.py	Sun Nov 19 14:29:17 2017 -0800
@@ -28,6 +28,3 @@
     """
     def getInteractiveFields(self):
         raise NotImplementedError()
-
-    def findContentFromPath(self, path):
-        raise NotImplementedError()