Make ParseLevel case-insensitive

Coming from an environment where loglevels are always specified in
uppercase, having ParseLevel be case-insensitive is a nice to have.

logrus.go

@@ -3,6 +3,7 @@ package logrus
 import (
 	"fmt"
 	"log"
+	"strings"
 )
 
 // Fields type, used to pass to `WithFields`.
@@ -33,7 +34,7 @@ func (level Level) String() string {
 
 // ParseLevel takes a string level and returns the Logrus log level constant.
 func ParseLevel(lvl string) (Level, error) {
-	switch lvl {
+	switch strings.ToLower(lvl) {
 	case "panic":
 		return PanicLevel, nil
 	case "fatal":

logrus_test.go

@@ -255,30 +255,58 @@ func TestParseLevel(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Equal(t, PanicLevel, l)
 
+	l, err = ParseLevel("PANIC")
+	assert.Nil(t, err)
+	assert.Equal(t, PanicLevel, l)
+
 	l, err = ParseLevel("fatal")
 	assert.Nil(t, err)
 	assert.Equal(t, FatalLevel, l)
 
+	l, err = ParseLevel("FATAL")
+	assert.Nil(t, err)
+	assert.Equal(t, FatalLevel, l)
+
 	l, err = ParseLevel("error")
 	assert.Nil(t, err)
 	assert.Equal(t, ErrorLevel, l)
 
+	l, err = ParseLevel("ERROR")
+	assert.Nil(t, err)
+	assert.Equal(t, ErrorLevel, l)
+
 	l, err = ParseLevel("warn")
 	assert.Nil(t, err)
 	assert.Equal(t, WarnLevel, l)
 
+	l, err = ParseLevel("WARN")
+	assert.Nil(t, err)
+	assert.Equal(t, WarnLevel, l)
+
 	l, err = ParseLevel("warning")
 	assert.Nil(t, err)
 	assert.Equal(t, WarnLevel, l)
 
+	l, err = ParseLevel("WARNING")
+	assert.Nil(t, err)
+	assert.Equal(t, WarnLevel, l)
+
 	l, err = ParseLevel("info")
 	assert.Nil(t, err)
 	assert.Equal(t, InfoLevel, l)
 
+	l, err = ParseLevel("INFO")
+	assert.Nil(t, err)
+	assert.Equal(t, InfoLevel, l)
+
 	l, err = ParseLevel("debug")
 	assert.Nil(t, err)
 	assert.Equal(t, DebugLevel, l)
 
+	l, err = ParseLevel("DEBUG")
+	assert.Nil(t, err)
+	assert.Equal(t, DebugLevel, l)
+
 	l, err = ParseLevel("invalid")
 	assert.Equal(t, "not a valid logrus Level: \"invalid\"", err.Error())
 }