Add redis and file options to config file

To customize redis and file configurations, this commit adds the "redis" and "file" options. We can write options to a config file as follows:

```
# DO NOT check this file into VCS (e.g. git).
# Parallelization setting differs between machines.
no_gc: true
ipc: redis         # 'file' or 'redis', if 'redis', gem and server are required
sort_by: time     # 'time' or 'size',  if 'time', store execution time and use it next time
workers: 2        # half of cpu - number of cpu
spork_port: 9240  # specified port..port+N-1 are used
redis:
    host: localhost
    port: 6379
    namespace: qspec
    driver: hiredis
file:
    path: '/tmp/qspec'
```