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 `mermaid-js#333`, the
same value used in `src/themes/flowchart.scss`.

Closes mermaid-js#885

src/diagrams/flowchart/flowRenderer.js

@@ -78,6 +78,7 @@ export const addVertices = function (vert, g, svgId) {
         tspan.setAttributeNS('http://www.w3.org/XML/1998/namespace', 'xml:space', 'preserve')
         tspan.setAttribute('dy', '1em')
         tspan.setAttribute('x', '1')
+        tspan.setAttribute('fill', '#333')
         tspan.textContent = rows[j]
         svgLabel.appendChild(tspan)
       }