Rename :style to :properties in cell hash (style will continue to work for a while)

I've been unhappy with my poor name choice for this for a while, so this is to fix that. `style` will continue to work for several versions (probably version 3 or 4) but we will document with `properties`.

Feedback welcome.

```ruby
def table_data
  [{
    cells: [{
      title: "My title",
      properties: { # Used to be `style:`
        any_writable_property: "My value"
      }
    }]
  }]
end
```

CC: issue #526.