Use viper for configuration

[jlewi]

It might be nice if we could support configuring the GitHubApp from a configuration file rather than having a huge number of command line flags; like Palantir GitHubApps
https://github.com/palantir/go-githubapp/blob/develop/example/config.yml

Is viper a good solution for dealing with this?
https://github.com/spf13/viper

[jlewi]

The other advantage of using viper is that when we use hydros as a CLI the configuration could be stored in the home directory and people could configure specific settings once for themselves; e.g. the path of their GitHubAppKey and the appid