Better String() output.

[flowchartsman]

It would be nice to be able to use %q in conf.String() to get clearer output for strings and slices, but when used indiscriminately against the example, the results are less than ideal for certain types:

SUB_VALUE='\x01' TIME_TO_WAIT="5s" DNS_SERVER="1.1.1.1" DEBUG=%!q(bool=false) DB_SERVERS=["127.0.0.1" "127.0.0.2"]

DB_SERVERS and the other string values look good, but integers like SUB_VALUE are not great, and booleans like DEBUG simply don't render with %q at all. This could require a new function that attempts to crawl the structure looking for a string at some level, or a generification and code sharing of the type identification code in usage.go