Add support for HTML5 number spinner as parameter input.

Small request from Kurt Meister. He actually wanted a slider, but a slider with a rebuildSolid() call would suck.

I'm not sure where the documentation lives, but here's some text and an example usage:

A float, int, number or text parameter is created by including the following object in the array returned by getParameterDefinitions():
```javascript
{
  name: 'width',
  type: 'float',                      // or 'text' or 'int'
  initial: 1.23,                      // optional, sets the initial value
                                      // NOTE: parameter "default" is deprecated
  caption: 'Width of the thingy:',    // optional, displayed left of the input field
                                      // if omitted, the 'name' is displayed (i.e. 'width')

}
```
The number parameter type will only work in HTML5 browsers and you should probably also include the standard attributes min, max and step, as in this example:

```javascript
// -- OpenJSCAD.org logo
function getParameterDefinitions() {
  return [
    { name: 'size', type: 'number', initial: 3, caption: "Size of the cube:", min: 2.0, max: 3.5, step: 0.1 }
  ];
}

function main(params) {
   return union(
      difference(
         cube({size: params.size, center: true}),
         sphere({r:2, center: true})
      ),
      intersection(
          sphere({r: 1.3, center: true}),
          cube({size: 2.1, center: true})
      )
   ).translate([0,0,1.5]).scale(10);
}
```