Read configuration options from nginx.conf

Initial spike to let repsheet read configuration options from nginx.conf. Includes a few minor style tweaks to more closely match style of other ngx modules.

Main context commands:

- `repsheet_redis_host localhost`
- `repsheet_redis_port 6379`
- `repsheet_redis_timeout 5000`
- `repsheet_redis_max_length 1000` - *not used yet*
- `repsheet_redis_expiry 1` - *not used yet*

Location context commands:

- `repsheet on|off`
- `repsheet_recorder on|off` - *not used yet*
- `repsheet_proxy_headers on|off` - *not used yet*
- `repsheet_action notify|block` - *not used yet*