Bibtextools

Added
- Added subcommand functionality. Available subcommands are `clean`, `combine`,
and `modernize`.
- Added `clean` functionality, that e.g., allows renaming duplicate IDs.
- Added `combine` functionality that allows combining multiple bib files into a
single one.
- The minimum required version of Python is 3.7.

Fixed
- Fix issue when using `--replace_ids` with author names that already contain
unicode characters in ASCII LaTeX format, e.g., \"{O} instead of Ö.

Added
- Script to clean bib files with the following features
- Page field is cleaned (en-dash `--` is used to separate numbers)
- Month field is cleaned (numbers are used instead of the old `jan` format)
- Eprint field is cleaned ("arXiv:" is stripped)
- Title field is cleaned (acronyms are shielded with curly brackets)
- Support for adding `primaryClass` field for arXiv articles
- Entry-IDs can be replaced with the scheme `<Author><year><firstWordOfTitle>`