Default LogWriter config

[kpgalligan]

Currently we use CommonWriter for the default, and provide platformLogWriter() as an expect to simplify loading each platform's "preferred" LogWriter. Using the platform default would probably make general sense, although I've found that NSLogWriter on iOS is more difficult to use when doing local debug in Xcode than CommonWriter.

I think either calling platformLogWriter(), or maybe adding a new function like devLogWriter(), and using CommonWriter instead of NSLogWriter on iOS, might be a better option for default. Android and JS certainly would be better off with their platform defaults.