Checkdmarc

-----

- Add `get_nameservers()` to the API
- Add `NS` record lookup to output as `ns`
- Add `--ns` option to CLI for listing approved nameservers

-----

- Fix `--mx` CLI option

-----

- Bugfix: STARTTLS caching
- Add MX warnings for parked domains
- Increase default DNS timeout from 2.0 seconds to 6.0 seconds

-----

- Bugfix: CSV format `-f csv` for starttls header
- Bugfix: Always properly close an SMTP connection
- Cache DNS and STARTTLS results in memory
- Use python3 in docs Makefile for Sphinx build
- Add `--debug` option
- Make warning about proper SPF records for MX hosts an only show with `--debug`
(Very noisy - Many hosts use DKIM without SPF to DMARC align bouncebacks)

-----

- Bugfix: Always raise warning when SPF type DNS records are found
- Add check for proper SPF records for MX hosts
- Add check for STARTTLS
- Add option `-p/--parked` to check for best practices for parked domains
- Add option `--mx` to provide a list of approved MX hostnames
- Add `query_bimi_record()` to the API

-----

- Fix parsing of TXT records over multiple lines (PR 36)