Make customisable props more extensible

[AnSavvides]

Currently, we do the following in BaseChart:

this.showTooltips = this.props.showTooltips ? true : false;
this.transitionDuration = this.props.transitionDuration || 1000;

Let's instead move this information to an object like this:

const customisableProps = {
    showTooltips: true,
    transitionDuration: 1000
};

This way we'll have a default value per customisable prop and we can just add to that object when we want to add another prop.

We can then just iterate through each key and make the necessary assignments in the BaseChart constructor.