Accessor strings (ready)

Dimension accessors can be simple strings for common use cases.

Example:
```javascript
dimensions: {	
  x: function (d, i) { return d.ts; },
  y: function (d, i) { return d.idle; }
}
```
becomes
```javascript
dimensions : { x: 'ts', y: 'idle' }
```

And may be any level deep.
Example: ```'dc.ord.latency'```