(#2744) Add autosign command

This pull request adds support for using an external command to autosign certificates. If the `autosign_command` setting is specified, incoming CSRs will be run with that command. The specified command will be given the CSR name as the first argument, and the body of the CSR will be encoded in PEM format and passed to the command on stdin.

This implements part of GH-1522.