Allow text substitutions when generating slugs

The `slugify()` function used by Pelican is in general very good at
coming up with something both readable and URL-safe. However, there are
a few specific cases where it causes conflicts. One that I've run into
is using the strings `C++` and `C` as tags, both of which transform to
the slug `c`. This commit adds an optional `SLUG_SUBSTITUTIONS` setting
which is a list of 2-tuples of substitutions to be carried out
case-insensitively just prior to stripping out non-alphanumeric
characters. This allows cases like `C++` to be transformed to `CPP` or
similar. This can also improve the readability of slugs.

pelican/tests/test_utils.py

@@ -94,6 +94,17 @@ class TestUtils(LoggedTestCase):
         for value, expected in samples:
             self.assertEqual(utils.slugify(value), expected)
 
+    def test_slugify_substitute(self):
+
+        samples = (('C++ is based on C', 'cpp-is-based-on-c'),
+                   ('C+++ test C+ test', 'cpp-test-c-test'),
+                   ('c++, c#, C#, C++', 'cpp-c-sharp-c-sharp-cpp'),
+                   ('c++-streams', 'cpp-streams'),)
+
+        subs = (('C++', 'CPP'), ('C#', 'C-SHARP'))
+        for value, expected in samples:
+            self.assertEqual(utils.slugify(value, subs), expected)
+
     def test_get_relative_path(self):
 
         samples = ((os.path.join('test', 'test.html'), os.pardir),