[vector-api] Friendly styling (work in progress)

This PR begins the implementation of a suggestion by @pagameba during the weekly hangout before Christmas.

In short, it allows you to pass strings instead of objects for simple styles. For example,
```javascript
new ol.style.Style({
  fill: new ol.style.Fill({
    color: 'rgba(255, 255, 255, 0.6)'
  }),
  stroke: new ol.style.Stroke({
    color: '#319FD3',
    width: 1
  })
}
```
becomes
```javascript
new ol.style.Style({
  fill: 'rgba(255, 255, 255, 0.6)',
  stroke: '#319FD3'
})
```

This is a work in progress, and it is not yet clear what other impact this will have on the codebase.

Comments welcome.