Add ability to continue pretty-printing deeper after encountering a stringer or error

With a Go program I am writing, I have stringers nested in stringers, and, using Spew for debugging, I decided it would be useful to have the ability to see them recursively.

This commit adds a new field to `ConfigState`, `ContinueOnMethod`, which specifies whether to continue when a stringer or error interface is encountered, as well as some code that causes `handleMethods` in common.go to return false and print the string/error value if ContinueOnMethod is true.