Envelope

– Address properties are "" if empty (handy when comparing strings)
- replace invalid characters when loading (the way the e-mail clients behave)
- headers truly case-insensitive when loading
- `.from_()` and `.sender()` truly return an `Address` even if not set (and then the Address is empty)
- fix: the internal cache correctly regenerated
- message is internally held in bytes -> helpful error message when charset fails

- exposed Address
- Address casefold method and `user` and `host` properties
- loading ignores invalid headers (and prints out a warning)

- fix case-insensitive header loading
- fix non-latin chars in header loading

- fix charset for base64 encoded content
- fix the default encoding while casting an attachment to string

- CHANGED:
* `.attach(filename=)` renamed to `.attach(name=)`
* `--attachment` renamed to `--attach`
* `.to()`, `.cc()`, `.bcc()`, `.from_()`, `.sender()`, `.reply_to()`, `.recipients()` now return an `Address` object(s), not `str`
- `.as_message` -> Message
- e-mail validation
- proper e-mail addresses handling: they can be in a string delimited by comma (or semicolon)
- text alternatives
- fix encoded headers
- fix attainable contents fetching
- embedding inline images
- loading parses attachments and decrypts
- contacts might be cleared (ex: to clear `To` header intuitively type `.to(False)`)
- `.attachments`, `--attachments [NAME]` -> read the list of the attachments
- preview returns nicely formatted headers

- fix: smtplib.SMTP_SSL handshake operation timed out caught
- if a line is longer than 1000 characters, makes the message be transferred safely by bytes (which would not break up DKIM)
- when "Content-Transfer-Encoding" is set to "base64" or "quo-pri" and reading message, it gets decoded (useful when loading EML files `cat file.eml | envelope --message`)
- preview method available from CLI