allow wildcards in files option

The files option currently only supports single files, you can't use file globs/wildcards.

In my latest project, every time I added a new html file I forgot to change the gruntfile.js so build was not including the new files. 

This allows to use wildcard in files option like this: 

```javascript
targethtml: {
    dist: {
        files: {
            "dist/": "app/*.html"
        }
    }
}

```