Use helpers for dealing with confusing "0"/"1" boolean values

This is an approach to fix #971. This adds `config_boolean_true?` and `config_boolean_false?` to wrap boolean field values. Due to the way services are currently stored in GitHub, these boolean config values will be passed as either "1", "0", or `nil`.

We've decided not to massage these into `true`/`false` values, since "1"/"0" is consistent with the hooks API responses.

**Note:** I'm not tied to these helper names but wanted to lean toward something pretty explicit. I'm definitely up for changing them if someone has any good ideas.

/cc @kdaigle
I started down that path initially until I ran across [these other config helpers](https://github.com/github/github-services/blob/master/lib/service/http_helper.rb#L25-L39) and ended up modeling my approach after them.

Either works for me but we should be consistent across the board if possible.