Implement measures and style overrides

[markusand]

Apparently there's possible option the allow initial measures (such as padding) override on initialization. https://github.com/frappe/charts/blob/7adc904b08fbb45fb30372d9c6a3c3df43f80085/src/js/charts/BaseChart.js#L83

Styling customization is the major flaw I see in this cute charting library. Allowing some svg parameters tune combined with some css variables override would help to overcome this limitation.