config: Refactor parsed option handling

Validate the parsed options directly after read and inform about errors.

cmd/micro/micro.go

@@ -273,6 +273,10 @@ func main() {
 				screen.TermMessage(err)
 				continue
 			}
+			if err = config.OptionIsValid(k, nativeValue); err != nil {
+				screen.TermMessage(err)
+				continue
+			}
 			config.GlobalSettings[k] = nativeValue
 			config.VolatileSettings[k] = true
 		}