[winston #1606]: stringify info.message in simple formatter

[DABH]

"Fixes" cases such as the one here where users want simple to stringify objects. Fair enough, "simple" sounds like it should "just work" even if jsonStringify is an extra performance hit.

winstonjs/winston#1606

[DABH]

@indexzero this causes info.message to get quotes around it if it's a string, e.g.
'info: "whatever"' instead of 'info: whatever'
do you reckon that's ok? jsonStringify and util.inspect both do that, not sure if one can get around it without explicit typecheck/case for String.