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_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())
 }