Rename Substr to Slice

That is whas was implemented, not Substr.

Also make the API more similar to Go's internal slice by making both the start and end indices optional.

See #990

tpl/template_test.go

@@ -276,7 +276,7 @@ func TestIn(t *testing.T) {
 	}
 }
 
-func TestSubstr(t *testing.T) {
+func TestSlice(t *testing.T) {
 	for i, this := range []struct {
 		v1     interface{}
 		v2     int
@@ -286,14 +286,17 @@ func TestSubstr(t *testing.T) {
 		{"abc", 1, 2, "b"},
 		{"abc", 1, 3, "bc"},
 		{"abc", 0, 1, "a"},
+		{"abcdef", -1, -1, "abcdef"},
+		{"abcdef", -1, 2, "ab"},
+		{"abcdef", 2, -1, "cdef"},
 		{123, 1, 3, "23"},
 		{tstNoStringer{}, 0, 1, false},
 	} {
-		result, err := Substr(this.v1, this.v2, this.v3)
+		result, err := Slice(this.v1, this.v2, this.v3)
 
 		if b, ok := this.expect.(bool); ok && !b {
 			if err == nil {
-				t.Errorf("[%d] Substr didn't return an expected error", i)
+				t.Errorf("[%d] Slice didn't return an expected error", i)
 			}
 		} else {
 			if err != nil {