Configure platforms in config file

[niklasmohrin]

There should be a config file option to specify what platforms should be used for looking up pages. It should take the same values as the --platform CLI argument. The precedence rules should be

1. If --platform passed, use these values
2. Otherwise, if config file specifies platforms, use these values
3. Otherwise, use PlatformType::current()

The handling of the common platform should (for now) not be changed. We can work out these details when tackling #323.