Checkdmarc

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- Check for TLSA records

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- Add support for parsing SMTP TLS Reporting ([RFC8460](https://www.rfc-editor.org/rfc/rfc8460.html)) DNS records

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- Fix DNSSEC test
- Add missing `import dns.dnssec`
- Always use the actual subdomain or domain provided (close 114)

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- Include MTA-STS and BIMI results in CSV output
- Renamed `include_dmarc_tag_descriptions` parameter in `checkdmarc.check_domains()` to `include_tag_descriptions`
- Added the `include_tag_descriptions` parameter to `checkdmarc.bimi.check_bimi()`
- Ignore encoding value when checking the `Content-Type` header during the MTA-STS policy download
- Added the exception class `MTASTSPolicyDownloadError`
- Update documentation

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- Major refactoring: Change from a single module to a package of modules, with each checked standard as its own package
- Add support for MTA-STS [RFC 8461](https://www.rfc-editor.org/rfc/rfc8461)
- Add support for [BIMI](https://www.ietf.org/archive/id/draft-brand-indicators-for-message-identification-04.html)
- Specify a BIMI selector using the `--bimi-selector`/`-b` option
- Various bug fixes

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- Fix SPF query error and warning messages
- More clear `fo` tag warning (PR 106)
- Do not raise a `DMAECRecordNotFound` exception when the `MultipleDMARCRecordsException` is raised (PR 108)
- Add support for null MX records - [RFC 7505](https://www.rfc-editor.org/rfc/rfc7505.html) (PR #109)