fix loop to work with different int types

`loop` when passed a scrach or $variable that contained an int as the
function was typed to only take int64. This changes it more like the
other functions that takes a interface{} and type switches/converts it
using reflect. Made it handle integer strings while I was at it.

Fixes #1143

template/funcs.go

@@ -697,16 +697,39 @@ func indent(spaces int, s string) (string, error) {
 // 			print(i)
 // 		}
 //
-func loop(ints ...int64) (<-chan int64, error) {
-	var start, stop int64
-	switch len(ints) {
+func loop(ifaces ...interface{}) (<-chan int64, error) {
+
+	to64 := func(i interface{}) (int64, error) {
+		v := reflect.ValueOf(i)
+		switch v.Kind() {
+		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32:
+			return int64(v.Int()), nil
+		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32:
+			return int64(v.Uint()), nil
+		case reflect.String:
+			return parseInt(v.String())
+		}
+		return 0, fmt.Errorf("loop: bad argument type: %T", i)
+	}
+
+	var i1, i2 interface{}
+	switch len(ifaces) {
 	case 1:
-		start, stop = 0, ints[0]
+		i1, i2 = 0, ifaces[0]
 	case 2:
-		start, stop = ints[0], ints[1]
+		i1, i2 = ifaces[0], ifaces[1]
 	default:
-		return nil, fmt.Errorf("loop: wrong number of arguments, expected 1 or 2"+
-			", but got %d", len(ints))
+		return nil, fmt.Errorf("loop: wrong number of arguments, expected "+
+			"1 or 2, but got %d", len(ifaces))
+	}
+
+	start, err := to64(i1)
+	if err != nil {
+		return nil, err
+	}
+	stop, err := to64(i2)
+	if err != nil {
+		return nil, err
 	}
 
 	ch := make(chan int64)

template/template_test.go

@@ -1249,11 +1249,34 @@ func TestTemplate_Execute(t *testing.T) {
 			"012",
 			false,
 		},
+		{
+			"helper_loop_start",
+			&NewTemplateInput{
+				Contents: `{{ range loop 1 3 }}1{{ end }}`,
+			},
+			&ExecuteInput{
+				Brain: NewBrain(),
+			},
+			"11",
+			false,
+		},
+		{
+			"helper_loop_text",
+			&NewTemplateInput{
+				Contents: `{{ range loop 1 "3" }}1{{ end }}`,
+			},
+			&ExecuteInput{
+				Brain: NewBrain(),
+			},
+			"11",
+			false,
+		},
 		{
 			// GH-1143
 			"helper_loop_var",
 			&NewTemplateInput{
-				Contents: `{{$n := 3 }}{{ range $i := loop $n }}{{ $i }}{{ end }}`,
+				Contents: `{{$n := 3 }}` +
+					`{{ range $i := loop $n }}{{ $i }}{{ end }}`,
 			},
 			&ExecuteInput{
 				Brain: NewBrain(),