Add optional encryption

[darkdragon-001]

Cryptography

• Use AES-GCM-256 for encryption
• Use PBKDF2 with SHA-256 and 100000 iterations for key derivation

Behavior

• A human-readable password will be automatically be generated when not set
• The password will be appended to the URI as fragment #password
• The IV (12 bytes) is prepended to the ciphertext
• Everything is base64 encoded (33% overhead)
• Update is disabled when encryption fails to avoid overwriting

Fixes #12