Fix text label colors for flow charts

When htmlLabels is set to `false` and a background fill color is used
for a node, the text label will inherit the fill color, hiding the
actual text. To fix this, explicitly set the fill color to #333.

Closes #885

src/diagrams/flowchart/flowRenderer.js

@@ -79,6 +79,7 @@ export const addVertices = function (vert, g, svgId) {
         tspan.setAttribute('dy', '1em')
         tspan.setAttribute('x', '1')
         tspan.textContent = rows[j]
+        tspan.setAttribute('fill', '#333')
         svgLabel.appendChild(tspan)
       }
       vertexNode = svgLabel