Make cfgutil.ExplicitString a fmt.Stringer.
Make cfgutil.ExplicitString a fmt.Stringer.

This allows the option to be passed to the fmt package for conversion
to a string.

jrick/dcrwallet,jrick/btcwallet,jrick/dcrwallet,decred/dcrwallet,decred/dcrwallet,jrick/btcwallet