Add slack plugin.

Respects <slack> blocks in the hydra config, with attributes:

* jobs: a regexp matching the job name (in the format project:jobset:job)
* url: The URL to a slack incoming webhook
* force: If true, always send messages. Otherwise, only when the build status changes

Multiple <slack> blocks are allowed

hydra-module.nix

@@ -20,6 +20,7 @@ let
     { NIX_REMOTE = "daemon";
       SSL_CERT_FILE = "/etc/ssl/certs/ca-certificates.crt";
       OPENSSL_X509_CERT_FILE = "/etc/ssl/certs/ca-certificates.crt"; # FIXME: remove on NixOS >= 15.07
+      PERL_LWP_SSL_CA_FILE = "/etc/ssl/certs/ca-certificates.crt";
       PGPASSFILE = "${baseDir}/pgpass";
     } // hydraEnv // cfg.extraEnv;