Allow loading remote files

I wanted to decide whether to load files either locally or from a cdn at build time. This pull will always allow external paths (i.e. `https://`, `http://`, or `//`) to validate.

```
<html>
<head>
    <title>grunt-html-build - Test Page</title>
   <!-- build:script bundle_remote -->
    <script type="text/javascript" src="//cdn.jsdelivr.net/jquery/2.1.0/jquery.js"></script>
    <script type="text/javascript" src="//cdn.jsdelivr.net/bootstrap/3.1.1/js/bootstrap.min.js"></script>
    <!-- /build -->
</head>
<body id="landing-page">
</body>
</html>
```
```
htmlbuild: {
    dist: {
        src: 'index.html',
        dest: 'samples/',
        options: {
            scripts: {
                bundle_remote: [
                    "//cdn.jsdelivr.net/jquery/2.1.0/jquery.js",
                    "//cdn.jsdelivr.net/bootstrap/3.1.1/js/bootstrap.min.js"
                ]
            }
        }
    }
}
```
Creates
```
<html>
    
    <head>
        <title>grunt-html-build - Test Page</title>
        <script type="text/javascript" src="//cdn.jsdelivr.net/jquery/2.1.0/jquery.js"></script>
        <script type="text/javascript" src="//cdn.jsdelivr.net/bootstrap/3.1.1/js/bootstrap.min.js"></script>
    </head>
    
    <body id="landing-page">
    </body>
</html>
```