When encrypting/signing a stream, this allows you to indicate whether it's a
stream of Strings or Uint8Arrays (using message.fromText or message.fromBinary,
respectively.)
When signing text, this allows you to control whether to create a cleartext
message or a regular armored text message.
When creating a detached signature, it allows you to control whether it's "meant
for" (verifying against) a cleartext message. A cleartext message has trailing
whitespace trimmed before signing. This fixes the case of passing a detached
signature from sign() to encrypt(). Since encrypt() doesn't create a cleartext
message, the signature would be invalid if the text contained lines with
trailing whitespace.
