allow type less which will use grunt-contrib-less to compile less files.

We had the need of replacing LESS block with less compilation result (css). 
This change will allow the use of type "less". Link tag will feed into grunt-contrib-less config for less compilation while script tag will be ignored.  

For example:

```html
    <!-- build:less less.css -->
    <link href="less1.less" rel="stylesheet/less">
    <link href="less2.less" rel="stylesheet/less">
    <script src="less.js" type="text/javascript"></script>
    <!-- endbuild -->
```

This will compile less and produce:
```html
    <link href="1234.less.css" rel="stylesheet/less">
```