addToPoint option for easier point-to-point modeling

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---

Proposal: add `addToPoint` option for items, and named `path` points for easier point-to-point modeling. Currently you have to copy `translate` values. Consider this model for an arm

``` js
let chest = new Zdog.Shape({
path: [
{ x: -4 },
{ x: 4 }
],
// ...
});

let upperArm = new Zdog.Shape({
addTo: chest,
path: [
{ y: 0 },
{ y: 3 },
],
// connect to shoulder
translate: { x: -4 },
// ...
});

let foreArm = new Zdog.Shape({
addTo: upperArm,
path: [
{ y: 0 },
{ y: 3 },
],
// connect to elbow
translate: { y: 3 },
// ...
});
// hand
new Zdog.Shape({
addTo: foreArm,
// connect to wrist
translate: { y: 3 },
// ...
});
```

The `translate` values for `upperArm`, `foreArm` and hand are all taken from a point of the item's parent. With an `addToPoint` option and named path point, you could directly set that value without have to copy over the values.

``` js
let chest = new Zdog.Shape({
path: [
{ x: -4, name: 'rightShoulder' },
{ x: 4 }
],
// ...
});

let upperArm = new Zdog.Shape({
addTo: chest,
addToPoint: 'rightShoulder',
path: [
{ y: 0 },
{ y: 3, name: 'elbow' },
],
// ...
});

let foreArm = new Zdog.Shape({
addTo: upperArm,
addToPoint: 'elbow',
path: [
{ y: 0 },
{ y: 3, name: 'wrist' },
],
// ...
});
// hand
new Zdog.Shape({
addTo: foreArm,
addToPoint: 'wrist',
// ...
});
```

Start at the Zdog.Shape entry point and trace how addTo, translate, and path points are handled. Compare the existing arm example with the proposed named-point model. Done means items can attach through addToPoint and named path points without manually copying parent translate values.

Written by the indexing model from the issue text.