feat: add util.mako_renderer() function

tests/files/bar/_sync.toml

@@ -0,0 +1,4 @@
+
+[files."baz"]
+renderer = 'mako'
+tags = ['baz']

tests/files/bar/baz

@@ -0,0 +1 @@
+machine_is_live = ${machine_is_live}

tests/test_util.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8; -*-
 
+import os
 from unittest import TestCase
 from unittest.mock import MagicMock
 
@@ -12,3 +13,14 @@ class TestExists(TestCase):
         c = MagicMock()
         mod.exists(c, '/foo')
         c.run.assert_called_once_with('test -e /foo', warn=True)
+
+
+class TestMakoRenderer(TestCase):
+
+    def test_basic(self):
+        c = MagicMock()
+        renderer = mod.mako_renderer(c, env={'machine_is_live': True})
+        here = os.path.dirname(__file__)
+        path = os.path.join(here, 'files', 'bar', 'baz')
+        rendered = renderer(path, vars={})
+        self.assertEqual(rendered, 'machine_is_live = True')