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 dictionary 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.