Handle 'true'/'false' stringified boolean options and better config error handling

[gitKrystan]

• Coerce 'true'/'false' into actual booleans (Fixes #223)
• Better error messages

Ideally, we'd use yargs for this, but doing so would involve changing codemod-cli to allow consumers to pass in yargs options here:

https://github.com/rwjblue/codemod-cli/blob/4a126ec9cd0ff4a482a69aa7f872ec629e23ba66/src/options-support.js#L14