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.
More precisely, group tags or categories without considering the case.
This fixes the bug where two categories with just the case as difference were
considered as distinct, but generate the same file: one overwriting the other.
Thanks to @Avaris for helping with the tests.
