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/settings.py

@@ -105,6 +105,7 @@ DEFAULT_CONFIG = {
     'PLUGINS': [],
     'TEMPLATE_PAGES': {},
     'IGNORE_FILES': ['.#*'],
+    'SLUG_SUBSTITUTIONS': (),
     }
 
 def read_settings(path=None, override=None):