Implement the functional options design pattern

[pd93]

Dave Cheney has recently popularised the following pattern for passing configuration options to Golang structures with private fields. We should investigate whether this is a good fit for PLog.

https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
https://play.golang.org/p/VcWqWcAEyz

[pd93]

Closed by 938465c