str.replace supports a short form that doesn't require the n argument, …

…closes #262

```
"aaaa".replace("a", "x") # "xxxx"
```

docs/types/string.md

@@ -337,19 +337,21 @@ Creates a new string, repeating the original one `i` times:
 "string".repeat(2) # "stringstring"
 ```
 
-### replace(x, y, n)
+### replace(x, y [, n])
 
 Replaces occurrences of `x` with `y`, `n` times.
-If `n` is negative it will replace all occurrencies:
+If `n` is omitted, or negative, it will replace all occurrencies:
 
 ``` bash
 "string".replace("i", "o", -1) # "strong"
+"aaaa".replace("a", "x") # "xxxx"
+"aaaa".replace("a", "x", 2) # "xxaa"
 ```
 
 You can also replace an array of characters:
 
 ``` bash
-"string".replace(["i", "g"], "o", -1) # "strono"
+"string".replace(["i", "g"], "o") # "strono"
 ```
 
 ### title()

evaluator/evaluator_test.go

@@ -895,7 +895,9 @@ c")`, []string{"a", "b", "c"}},
 		{`[1,2,3].slice(-1, 3)`, []int{3}},
 		{`[1,2,3].slice(-1, 1)`, []int{3}},
 		{`"a".replace("a", "b", -1)`, "b"},
+		{`"a".replace("a", "b")`, "b"},
 		{`"ac".replace(["a", "c"], "b", -1)`, "bb"},
+		{`"ac".replace(["a", "c"], "b")`, "bb"},
 		{`"a".str()`, "a"},
 		{`1.str()`, "1"},
 		{`[1].str()`, "[1]"},

evaluator/functions.go

@@ -1165,17 +1165,31 @@ func repeatFn(tok token.Token, args ...object.Object) object.Object {
 	return &object.String{Token: tok, Value: strings.Repeat(args[0].(*object.String).Value, int(args[1].(*object.Number).Value))}
 }
 
+// replace("abd", "d", "c") --> short form
 // replace("abd", "d", "c", -1)
 // replace("abc", ["a", "b"], "c", -1)
 func replaceFn(tok token.Token, args ...object.Object) object.Object {
-	err := validateArgs(tok, "replace", args, 4, [][]string{{object.STRING_OBJ}, {object.STRING_OBJ, object.ARRAY_OBJ}, {object.STRING_OBJ}, {object.NUMBER_OBJ}})
+	var err object.Object
+
+	// Support short form
+	if len(args) == 3 {
+		err = validateArgs(tok, "replace", args, 3, [][]string{{object.STRING_OBJ}, {object.STRING_OBJ, object.ARRAY_OBJ}, {object.STRING_OBJ}})
+	} else {
+		err = validateArgs(tok, "replace", args, 4, [][]string{{object.STRING_OBJ}, {object.STRING_OBJ, object.ARRAY_OBJ}, {object.STRING_OBJ}, {object.NUMBER_OBJ}})
+	}
+
 	if err != nil {
 		return err
 	}
 
 	original := args[0].(*object.String).Value
 	replacement := args[2].(*object.String).Value
-	n := int(args[3].(*object.Number).Value)
+
+	n := -1
+
+	if len(args) == 4 {
+		n = int(args[3].(*object.Number).Value)
+	}
 
 	if characters, ok := args[1].(*object.Array); ok {
 		for _, c := range characters.Elements {