Allow the ApolloEngine LogLevel to be set via the settings

[luhagel]

Some eslint formatting crept in, bloats the diff, but since it's just following the eslintrc, it should be done sooner or later anyway ;)

[SachaG]

Thanks! Before I accept this though, could you also document the setting by registering it with registerSetting?

For example:

registerSetting('apolloEngine.logLevel', 'INFO', 'Log level (one of INFO, DEBUG, ...');

[luhagel]

sure thing

[SachaG]

Awesome, thanks :)