Breaking changes:
- a new `format` option has been added to `openpgp.encrypt`, `sign` and
`encryptSessionKey` to select the format of the output message. `format`
replaces the existing `armor` option, and accepts three values:
* if `format: 'armor'` (default), an armored signed/encrypted message is
returned (same as `armor: true`).
* if `format: 'binary'`, a binary signed/encrypted message is returned (same
as `armor: false`).
* if `format: 'object'`, a Message or Signature object is returned (this was
not supported before).
This change is to uniform the output format selection across all top-level
functions (following up to #1345).
- All top-level functions now throw if unrecognised options are passed, to make
library users aware that those options are not being applied.
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