Organize global substitution code

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

docs/callbacks.py

@@ -0,0 +1,8 @@
+from __future__ import unicode_literals
+
+
+def get_source_read_callback(substitutions):
+    def global_substitution_function(app, docname, source):
+        for old, new in substitutions:
+            source[0] = source[0].replace(old, new)
+    return global_substitution_function