Added wrapper support

The wrapper marks the unprintable characters from each style tag.
This is very useful when debugging or outputing it in different terminals. You can use it to identify the unprintable characters or even to escape them.

Also added tests for it.
Added a description about it to the readme dile.
wq

readme.md

@@ -149,6 +149,26 @@ Levels are as follows:
 2. 256 color support
 3. Truecolor support (16 million colors)
 
+### chalk.wrapper
+
+The wrapper marks the unprintable characters.  
+A wrapper can be added to the styles, so you can escape characters or add marks to then.  
+By default, these wrappers are empty strings `""`.
+
+The wrappers object has two properties, `pre` and `post`.  
+For example:
+
+```js
+const ctx = new chalk.constructor({wrapper: {
+	pre: '>',
+	post: '<',
+}});
+
+ctx.red('foo') // outputs "><foo><"
+```
+
+This can be specially useful when escaping characters, using it into a _PS1_ string or debugging and outputing it into different terminals/TTYs.
+
 ### chalk.supportsColor
 
 Detect whether the terminal [supports color](https://github.com/chalk/supports-color). Used internally and handled for you, but exposed for convenience.