Stricter typings

[rslawik]

Should we introduce stricter types for some of the properties of GoogleChart?

• type: string could be replaced with the union of all keys (string literals) from CHART_TYPES. That would prevent typos but it would require a way to extend that union to support arbitrary visualization types (similarly to HTMLElementTagNameMap). Suggested by @rictic.

• options: {}|undefined could be replaced with the union of all known options types from DefinitelyTyped google.visualization. For example: LineChartOptions|ColumnChartOptions.

• cols: unknown[]|undeifned could be use DataObjectColumn

• rows: unknown[][]|undefined could be use DataObjectRow

All those changes are likely to be breaking. We should weight the benefits they offer.