Split reader/writer implementation

[ryandbair]

Sometimes an application only needs to produce or consume CSV, but not both.

Splitting the converter trait into a reader and a writer trait would allow applications which need to work with custom types (but only in one direction) to avoid writing unnecessary code or leaving methods unimplemented. Converter would still stick around as a combination of reader and writer.

If there's interest, I'll implement this when I have some free time.