Add boolean config accessor

`Config.enable_foo` would return the string `"true"` or `"false"`
`Config.enable_foo?` would return `true` or `false`

As proposed in https://github.com/interagent/pliny-template/pull/108#issuecomment-45910189

I tried to write a test but I was unable, as it seems like I can't stub constants with `rr`