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/utils.py

@@ -231,7 +231,7 @@ class pelican_open(object):
         pass
 
 
-def slugify(value):
+def slugify(value, substitutions=()):
     """
     Normalizes string, converts to lowercase, removes non-alpha characters,
     and converts spaces to hyphens.
@@ -249,8 +249,10 @@ def slugify(value):
     if isinstance(value, six.binary_type):
         value = value.decode('ascii')
     # still unicode
-    value = unicodedata.normalize('NFKD', value)
-    value = re.sub('[^\w\s-]', '', value).strip().lower()
+    value = unicodedata.normalize('NFKD', value).lower()
+    for src, dst in substitutions:
+        value = value.replace(src.lower(), dst.lower())
+    value = re.sub('[^\w\s-]', '', value).strip()
     value = re.sub('[-\s]+', '-', value)
     # we want only ASCII chars
     value = value.encode('ascii', 'ignore')