add pretty format output support

Add pretty format output support, hope it will make `gulp-angular-templatecache` better.

_How to use it?_
set option `pretty` to true, when you initialize a `gulp-angular-templatecache`:
```javascript
gulp
    .src('pages/**/*.html')
    .pipe(templateCache({ pretty: true }))
    .pipe(gulp.dest('./build'));
```
then you'll get pretty format results, 
```javascript
angular.module("templates").run(["$templateCache", function($templateCache) {
  $templateCache.put("template1.html",
    "<div style=\"min-height:550px\">\n" +
    "    <div class=\"container-fluid\">\n" +
    "        <div class=\"row-fluid\">\n" +
    "            <div class=\"span9\">\n" +
    "                <div id=\"map\" class=\"img-rounded img-polaroid\" style=\"height:600px\"></div>\n" +
    "            </div>\n" +
    "            <div class=\"span3\">\n" +
    "                messages\n" +
    "            </div>\n" +
    "        </div>\n" +
    "    </div>\n" +
        ...
    "</div>\n" +
    ""
  );

  $templateCache.put("template2.html",
    "<div style=\"min-height:550px\">\n" +
    "    <div class=\"container-fluid\">\n" +
    "        <div class=\"row-fluid\">\n" +
    "            <div class=\"span9\">\n" +
    "                <div id=\"map\" class=\"img-rounded img-polaroid\" style=\"height:600px\"></div>\n" +
    "            </div>\n" +
    "            <div class=\"span3\">\n" +
    "                messages\n" +
    "            </div>\n" +
    "        </div>\n" +
    "    </div>\n" +
        ...
    "</div>\n" +
    ""
  );
  ...
}]);
```

Can you help review and give your suggestions?