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.

cypress/integration/rendering/flowchart-v2.spec.js

@@ -0,0 +1,31 @@
+/* eslint-env jest */
+import { imgSnapshotTest } from '../../helpers/util';
+
+describe('Flowchart v2', () => {
+  it('1: should render a simple flowchart', () => {
+    imgSnapshotTest(
+      `flowchart TD
+      A[Christmas] -->|Get money| B(Go shopping)
+      B --> C{Let me think}
+      C -->|One| D[Laptop]
+      C -->|Two| E[iPhone]
+      C -->|Three| F[fa:fa-car Car]
+      `,
+      {}
+    );
+  });
+
+  it('2: should render a simple flowchart with diagramPadding set to 0', () => {
+    imgSnapshotTest(
+      `flowchart TD
+      A[Christmas] -->|Get money| B(Go shopping)
+      B --> C{Let me think}
+      %% this is a comment
+      C -->|One| D[Laptop]
+      C -->|Two| E[iPhone]
+      C -->|Three| F[fa:fa-car Car]
+      `,
+      { flowchart: { diagramPadding: 0 } }
+    );
+  });
+});