tpl/lang: Add NumFmt function

NumFmt formats a number with a given precision using the requested
decimal, grouping, and negative characters.

Fixes #1444

tpl/lang/lang_test.go

@@ -0,0 +1,54 @@
+package lang
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/spf13/hugo/deps"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNumFormat(t *testing.T) {
+	t.Parallel()
+
+	ns := New(&deps.Deps{})
+
+	cases := []struct {
+		prec  int
+		n     float64
+		runes string
+
+		want string
+	}{
+		{2, -12345.6789, "", "-12,345.68"},
+		{2, -12345.6789, "- . ,", "-12,345.68"},
+		{2, -12345.1234, "- . ,", "-12,345.12"},
+
+		{2, 12345.6789, "- . ,", "12,345.68"},
+		{0, 12345.6789, "- . ,", "12,346"},
+		{11, -12345.6789, "- . ,", "-12,345.67890000000"},
+
+		{3, -12345.6789, "- ,", "-12345,679"},
+		{6, -12345.6789, "- , .", "-12.345,678900"},
+
+		// Arabic, ar_AE
+		{6, -12345.6789, "‏- ٫ ٬", "‏-12٬345٫678900"},
+	}
+
+	for i, c := range cases {
+		errMsg := fmt.Sprintf("[%d] %v", i, c)
+
+		var s string
+		var err error
+
+		if len(c.runes) == 0 {
+			s, err = ns.NumFmt(c.prec, c.n)
+		} else {
+			s, err = ns.NumFmt(c.prec, c.n, c.runes)
+		}
+
+		require.NoError(t, err, errMsg)
+		assert.Equal(t, c.want, s, errMsg)
+	}
+}