Change randomness behaviour to be like it prior to go1.20

Pre go1.20, rand.Seed needed to be called for randomness. Meaning, that
math/rand delivered deterministic values by default. This was changed
however. To restore the old behaviour, we need to manually seed our
random generator with a constant.

Additionally, this introduce a local instance of rand.Rand, in order
to prevent unexpected behaviour with other code that relies on the
global math/rand package.

Fixes dustinkirkland#10 (The readme and docs explain this, therefore I consider this
solved, as it is by design and not a bug)

Fixes dustinkirkland#15

README.md

@@ -94,13 +94,15 @@ var (
         separator = flag.String("separator", "-", "The separator between words in the pet name")
 )
 
-func init() {
-        rand.Seed(time.Now().UTC().UnixNano())
-}
-
 func main() {
         flag.Parse()
-        rand.Seed(time.Now().UnixNano())
+
+        // To have a predictable series of petnames. (This is the default)
+        petname.Seed(1)
+
+        // To have an unpredictable series of petnames.
+        petname.NonDeterministicMode()
+
         fmt.Println(petname.Generate(*words, *separator))
 }
 ```

petname.go

@@ -41,24 +41,36 @@ var (
 
 // End word lists
 
-// Call this function once before using any other to get real random results
+var localRand *rand.Rand = rand.New(rand.NewSource(1))
+
+// NonDeterministicMode configures the local random generator used internally
+// to provide non deterministic results, instead of a pre-defined order that is
+// reproducible even after a process restart. If you wish to specify a custom
+// contant, call [Seed(int64)].
 func NonDeterministicMode() {
-	rand.Seed(time.Now().UnixNano())
+	localRand.Seed(time.Now().UnixNano())
+}
+
+// Seed configures the local random generator, allowing you to specify a
+// constant for reproducible "randomness" or provide a custom value for
+// "true" randomness.
+func Seed(seed int64) {
+	localRand.Seed(seed)
 }
 
 // Adverb returns a random adverb from a list of petname adverbs.
 func Adverb() string {
-	return adverbs[rand.Intn(len(adverbs))]
+	return adverbs[localRand.Intn(len(adverbs))]
 }
 
 // Adjective returns a random adjective from a list of petname adjectives.
 func Adjective() string {
-	return adjectives[rand.Intn(len(adjectives))]
+	return adjectives[localRand.Intn(len(adjectives))]
 }
 
 // Name returns a random name from a list of petname names.
 func Name() string {
-	return names[rand.Intn(len(names))]
+	return names[localRand.Intn(len(names))]
 }
 
 // Generate generates and returns a random pet name.