v0.0.43: Limited key pinning support. Stateless challenge support.

An existing key can now be specified in a targetfile by specifying

  request:
    key:
      id: key-id

where key-id is the key ID of the key. If the key is not found
or this option is not specified, a new key will be generated as usual.

Support for stateless challenges has been added. This mainly requires
appropriate webserver configuration. A command has been added to easily
retrieve the account key thumbprint to enable easy configuration of
stateless challenges.

The 'finished' quickstart dialog is now always printed to stdio
rather than shown via a dialog. It requires no acknowledgement and
because it is printed to stdio, can remain in view after acmetool
has exited, and is thus more helpful to the user.

stdio prompts are now word wrapped to 70 characters.