Add LevelFromString() helper

Author: disq

README.md

@@ -49,3 +49,22 @@ k := picolo.NewFrom(l, "[more-prefix]")
 k.Errorf("Error message: %v", err)
 //  2017-12-21 23:24:25.267 ERROR [some-prefix] [more-prefix] Error message: No such file or directory
 ```
+
+# Helpers
+
+Use `picolo.LevelFromString` to parse a string into a log level. This can be used like:
+
+```go
+// ...
+l := flag.String("logLevel", "debug", "Log level")
+flag.Parse()
+
+lvl, err := picolo.LevelFromString(*l)
+if err != nil {
+	// Unknown log level
+}
+
+logger := picolo.New(picolo.WithLevel(lvl))
+logger.Infof("One two three")
+// ...
+```

log.go

@@ -13,17 +13,19 @@ type Level int
 
 const (
 	// Log levels
-	LevelDebug   Level = iota // Debug level
-	LevelInfo                 // Info level
-	LevelWarning              // Warning level
-	LevelError                // Error level
+	LevelDebug   Level = iota + 1 // Debug level
+	LevelInfo                     // Info level
+	LevelWarning                  // Warning level
+	LevelError                    // Error level
 )
 
 const DefaultTimeFormat = "2006-01-02 15:04:05.000"
 
+var ErrUnknownLevel = fmt.Errorf("Unknown log level")
+
 // Logger is our logger struct
 type Logger struct {
-	mu sync.Mutex // used to synchronize output as well
+	mu   sync.Mutex // used to synchronize output as well
 	opts *options
 }
 
@@ -167,3 +169,19 @@ func (l Level) String() string {
 		return "UNKNOWN"
 	}
 }
+
+// LevelFromString returns the log level from string representation
+func LevelFromString(s string) (Level, error) {
+	switch s {
+	case "debug":
+		return LevelDebug, nil
+	case "info":
+		return LevelInfo, nil
+	case "warning":
+		return LevelWarning, nil
+	case "error":
+		return LevelError, nil
+	default:
+		return 0, ErrUnknownLevel
+	}
+}