fix(bytes): change Bytes function to accept string input but return a byte slice

The old method signature was just nonsensical, as you would always be
providing indented values via a string literal. So it makes much more
sense to have all methods accept a string argument, and then return
different types.

This also allows use of a `Bytesf` method.

This is technically a breaking change, but I'm classifying it as a
bugfix cause the old method signature was basically useless.

undent_example_test.go

@@ -7,11 +7,11 @@ import (
 )
 
 func ExampleBytes() {
-	b := undent.Bytes([]byte(`
+	b := undent.Bytes(`
 		{
 		  "hello": "world"
 		}`,
-	))
+	)
 
 	fmt.Println(string(b))
 	// Output:
@@ -20,6 +20,20 @@ func ExampleBytes() {
 	// }
 }
 
+func ExampleBytesf() {
+	s := undent.Bytesf(`
+		{
+		  "hello": "%s"
+		}`,
+		"world",
+	)
+	fmt.Println(string(s))
+	// Output:
+	// {
+	//   "hello": "world"
+	// }
+}
+
 func ExampleString() {
 	s := undent.String(`
 		{