allow custom template, add one more template

This is a follow up to davidtheclark/grunt-spritesmith-hd#9
It allowes for custom templates to be used which gives more flexibility.

It adds one template called scss_combined_maps-hd that can be used together with the template from https://github.com/benib/json2css_scss_combined_maps to get a result like:
````
$icons-ld: (
  icon: (
    x: 62px,
    y: 0px,
    offset_x: -62px,
    offset_y: 0px,
    width: 102px,
    height: 65px,
    total_width: 347px,
    total_height: 174px,
    image: '/images/ld-icons.png',
    hd-name: hd-icon
  ),
  ...
);
```

This allowes for the icon name to be a string instead of a variable, which makes it compatible with the compass spriting system and allowes for a painless transition to grunt-spritesmith. Another possible advantage is, that strings can be generated while variable names can not in sass. For a longer discussion about this, see twolfson/json2css#27.