Make tags and cats case insensitive. Fixes #704.

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.
Merge pull request #845 from Rogdham/case-insensitive_caterogy_tag

Make tags and cats case insensitive. Fixes #704.