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Fixes #708 SUMMARY_MAX_LENGTH=0 should return empty string
Merge pull request #867 from avaris/summary-max-length

Fixes #708 SUMMARY_MAX_LENGTH=0 should return empty string
Add warning to docs re: DELETE_OUTPUT_DIRECTORY
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.