Create a file that contains options

Bonus: remove unused variables
Fixes #7

Produces that (example)-output:

```
{
  "js": [
    "bootstrap-transition.js",
    "bootstrap-modal.js",
    "bootstrap-dropdown.js",
    "bootstrap-scrollspy.js",
    "bootstrap-tab.js",
    "bootstrap-tooltip.js",
    "bootstrap-popover.js",
    "bootstrap-affix.js"
  ],
  "css": [
    "reset.less",
    "scaffolding.less",
    "grid.less",
    "layouts.less",
    "type.less",
    "code.less",
    "labels-badges.less"
  ],
  "img": [
    "glyphicons-halflings.png",
    "glyphicons-halflings-white.png"
  ],
  "vars": {
    "@inputBorderRadius": "5px"
  }
}
```