* Implement CFSSL issuer plugin
Implement a Lemur plugin for generating certificates from the open
source certificate authority CFSSL
(https://github.com/cloudflare/cfssl). The plugin interacts with CFSSL
through the CFSSL REST API. The CFSSL configuration is defined in the
lemur.conf.py property file using property names prefixed with "CFSSL_".
* Update documentation to include CFSSL plugin
