v0.4.0

• New cryptoblob structure:

                     512B          0+B
                 +----------+---------------+
                 | comments | file contents |
                 +----------+---------------+
  16B     0+B    |        plaintext         | 64B     0+B     16B
+------+---------+--------------------------+-----+---------+------+
| salt | padding |        ciphertext        | MAC | padding | salt |
+------+---------+--------------------------+-----+---------+------+
|  urandom data  |      random-looking data       |  urandom data  |
+----------------+--------------------------------+----------------+

• Use new cipher: ChaCha20-IETF instead of SHAKE256-based cipher.
• Add new dependencies: pynacl (provides Argon2) and pycryptodomex (provides ChaCha20).
• Use a new KDF: Argon2 instead of scrypt and catpig.
• Rename: mode to action, hide to embed, unhide to extract.
• Compare MAC tags in costant time using hmac.compare_digest().
• Do not encrypt MAC tags.
• Forbid unknown CLI options.
• Do not convert specified file paths (input, output, keyfiles) to realpath before opening fd.
• Update logo.
• Other minor improvements.