Add "replace" function and documentation

This commit adds a `replace` function which takes three arguments - an
original string, the substring to replace, and the string with which to
replace it. This is of particular use when generating node names from IP
addresses where the node name may not contain "." characters.

The `strings.Replace` function in the Go standard library seems like it
would be better named `strings.ReplaceN`, so we wrap it to replace all
occurences of the string. If necessary, a gomplate function named
`replaceN` could be introduced calling `strings.Replace` directly.

README.md

@@ -46,6 +46,7 @@ Gomplate is an alternative that will let you process templates which also includ
 		- [`slice`](#slice)
 		- [`split`](#split)
 		- [`splitN`](#splitn)
+		- [`replace`](#replace)
 		- [`title`](#title)
 		- [`toLower`](#tolower)
 		- [`toUpper`](#toupper)
@@ -355,6 +356,16 @@ $ gomplate -i '{{ range splitN "foo:bar:baz" ":" 2 }}{{.}}{{end}}'
 foo
 bar:baz
 ```
+#### `replace`
+
+Replaces all occurrences of a given string with another.
+
+##### Example
+
+```console
+$ gomplate -i '{{ replace "172.21.1.42" "." "-" }}'
+172-21-1-42
+```
 
 #### `title`
 

gomplate.go

@@ -39,6 +39,7 @@ func (g *Gomplate) RunTemplate(text string, out io.Writer) {
 func NewGomplate(data *Data, leftDelim, rightDelim string) *Gomplate {
 	env := &Env{}
 	typeconv := &TypeConv{}
+	stringfunc := &StringFunc{}
 	ec2meta := aws.NewEc2Meta()
 	ec2info := aws.NewEc2Info()
 	return &Gomplate{
@@ -65,6 +66,7 @@ func NewGomplate(data *Data, leftDelim, rightDelim string) *Gomplate {
 			"contains":         strings.Contains,
 			"hasPrefix":        strings.HasPrefix,
 			"hasSuffix":        strings.HasSuffix,
+			"replace":          stringfunc.Replace,
 			"split":            strings.Split,
 			"splitN":           strings.SplitN,
 			"title":            strings.Title,

stringfunc.go

@@ -0,0 +1,10 @@
+package main
+
+import "strings"
+
+// StringFunc - string manipulation function wrappers
+type StringFunc struct {}
+
+func (t StringFunc) Replace(s, old, new string) string {
+	return strings.Replace(s, old, new, -1)
+}

stringfunc_test.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"testing"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestReplace(t *testing.T) {
+	sf := &StringFunc{}
+
+	assert.Equal(t, "Replaced",
+		sf.Replace("Orig", "Orig", "Replaced"))
+	assert.Equal(t, "ReplacedReplaced",
+		sf.Replace("OrigOrig", "Orig", "Replaced"))
+}