feature: configurable vertical segments

Add ability to create vertical grid.
Screenshot of resulting chart:
[![Screenshot](https://drive.google.com/uc?export=view&id=0B65dEGRmB3ViaHA5U2JERHFmYkE)](https://docs.google.com/file/d/0B65dEGRmB3ViaHA5U2JERHFmYkE/edit)
With next element added to config object:
```
...
        verticalGrid: {
          condition: function(index) {
            return index % 2
          },
          color: '#888',
          opacity: '0.2'
        }
...
```