Adds support for themes functions

With this changes the themes can have (not obligatory) a functions.py
file bundled with the templates. The purpose of this file is to hold
helper functions to be injected into the jinja2 templates. To control
which functions will be injected is specified the __export__ list. A
sample was inserted in pelican/themes/simple/templates/functions.py.

pelican/generators.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals, print_function
 
 import os
+import imp
 import math
 import random
 import logging
@@ -62,6 +63,14 @@ class Generator(object):
             ]),
             extensions=self.settings['JINJA_EXTENSIONS'],
         )
+        try:
+            theme_functions = imp.load_source('theme_functions', self._templates_path[0]+'/functions.py')
+        except IOError:
+            theme_functions = None
+
+        if theme_functions and '__export__' in vars(theme_functions):
+            for func_name in theme_functions.__export__:
+                self.env.globals[func_name] = getattr(theme_functions, func_name)
 
         logger.debug('template list: {0}'.format(self.env.list_templates()))
 

pelican/themes/simple/templates/functions.py

@@ -0,0 +1,9 @@
+# encoding: utf-8
+
+__export__ = ["helperFunction"]
+
+def helperFunction():
+    return "<p>I'm from a helper function!</p>"
+
+def anotherHelper():
+    return "I'm not being exported!"