Add --debug option to be improved on over time

Why:

 * first time using hugo I got very little info from --verbose output
   but I noticed there is quite a lot of useful DEBUG logging
 * asked for in other issues like https://github.com/gohugoio/hugo/issues/3514

This change addreses the need by:

 * adding a simple --debug flag which simply turns on debug level in stdout
   and logoutput if enabled.

hugolib/config.go

@@ -132,4 +132,5 @@ func loadDefaultSettingsFor(v *viper.Viper) {
 	v.SetDefault("enableGitInfo", false)
 	v.SetDefault("ignoreFiles", make([]string, 0))
 	v.SetDefault("disableAliases", false)
+	v.SetDefault("debug", false)
 }