Make all `read*` functions accept an options object, so that we can add config
options to them later (for #1166). This is necessary so that we can remove the
global `openpgp.config`, which doesn't work that well when importing
individual functions.
Furthermore, merge `readMessage` and `readArmoredMessage` into one function,
et cetera.
Instead of as modules.
Replace *.read with read*, *.readArmored with readArmored*, etc.
Replace cleartext.readArmored with readArmoredCleartextMessage.
Replace message.fromText with Message.fromText, etc.
openpgp.encrypt, sign, encryptSessionKey, encryptKey and decryptKey now
return their result directly without wrapping it in a "result" object.
Also, remove the `detached` and `returnSessionKey` options of
openpgp.encrypt.
