Fix undefined and null interpolated expressions (fixes #194)

Author: Qix-

index.js

@@ -207,8 +207,8 @@ function chalkTag(chalk, strings) {
 	const parts = [strings.raw[0]];
 
 	for (let i = 1; i < strings.length; i++) {
-		parts.push(args[i - 1].toString().replace(/[{}\\]/g, '\\$&'));
-		parts.push(strings.raw[i]);
+		parts.push(String(args[i - 1]).replace(/[{}\\]/g, '\\$&'));
+		parts.push(String(strings.raw[i]));
 	}
 
 	return template(chalk, parts.join(''));

test/template-literal.js

@@ -161,3 +161,9 @@ test('should not parse upper-case escapes', t => {
 	const ctx = m.constructor({level: 0});
 	t.is(ctx`\N\n\T\t\X07\x07\U000A\u000A\U000a\u000a`, 'N\nT\tX07\x07U000A\u000AU000a\u000A');
 });
+
+test('should properly handle undefined template interpolated values', t => {
+	const ctx = m.constructor({level: 0});
+	t.is(ctx`hello ${undefined}`, 'hello undefined');
+	t.is(ctx`hello ${null}`, 'hello null');
+});