Add loop template function to allow interation in templating.

The loop function is helpful as it allows fixed number
interations which may be desirable to users. The change also
includes a documentation update to show loop usgae.

Closes #201

docs/templates.md

@@ -99,6 +99,40 @@ Render:
 localhost:3306
 ```
 
+#### loop
+
+Accepts varying parameters and differs its behavior based on those parameters as detailed below.
+
+If loop is given a signle int input, it will loop up to, but not including the given integer from index 0:
+
+Example:
+```
+[[ range $i := loop 3 ]]
+this-is-loop[[ $i ]][[ end ]]
+```
+
+Render:
+```
+this-is-output0
+this-is-output1
+this-is-output2
+```
+
+If given two integers, this function will begin at the first integer and loop up to but not including the second integer:
+
+Example:
+```
+[[ range $i := loop 3 6 ]]
+this-is-loop[[ $i ]][[ end ]]
+```
+
+Render:
+```
+this-is-output3
+this-is-output4
+this-is-output5
+```
+
 #### timeNow
 
 Returns the current ISO_8601 standard timestamp as a string in the timezone of the machine the rendering was triggered on.

template/funcs.go

@@ -2,6 +2,7 @@ package template
 
 import (
 	"errors"
+	"fmt"
 	"text/template"
 	"time"
 
@@ -16,6 +17,7 @@ func funcMap(consulClient *consul.Client) template.FuncMap {
 		"consulKey":          consulKeyFunc(consulClient),
 		"consulKeyExists":    consulKeyExistsFunc(consulClient),
 		"consulKeyOrDefault": consulKeyOrDefaultFunc(consulClient),
+		"loop":               loop,
 		"timeNow":            timeNowFunc,
 		"timeNowUTC":         timeNowUTCFunc,
 		"timeNowTimezone":    timeNowTimezoneFunc(),
@@ -94,6 +96,30 @@ func consulKeyOrDefaultFunc(consulClient *consul.Client) func(string, string) (s
 	}
 }
 
+func loop(ints ...int64) (<-chan int64, error) {
+	var start, stop int64
+	switch len(ints) {
+	case 1:
+		start, stop = 0, ints[0]
+	case 2:
+		start, stop = ints[0], ints[1]
+	default:
+		return nil, fmt.Errorf("loop: wrong number of arguments, expected 1 or 2"+
+			", but got %d", len(ints))
+	}
+
+	ch := make(chan int64)
+
+	go func() {
+		for i := start; i < stop; i++ {
+			ch <- i
+		}
+		close(ch)
+	}()
+
+	return ch, nil
+}
+
 func timeNowFunc() string {
 	return time.Now().Format("2006-01-02T15:04:05Z07:00")
 }