Add flowchart.diagramPadding config option

This option lets you customise the padding around the whole flowchart.
It is equivalent to `er.diagramPadding` and defaults to 8px for
backwards compatibility with the current hardcoded value.

src/mermaidAPI.js

@@ -577,6 +577,7 @@ export default mermaidAPI;
  *     arrowMarkerAbsolute:false,
  *
  *     flowchart:{
+ *       diagramPadding:8,
  *       htmlLabels:true,
  *       curve:'linear',
  *     },