hydra-module: add config.extraEnv

This makes it easy to set environment variables for the Hydra server
(for example, your configuration.nix can use readFile to read an API
token to upload build results somewhere).

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Merge pull request #160 from thoughtpolice/extraEnv

hydra-module: add config.extraEnv