Add background-color to styles (#211)

Removed default background color, cosmetics
Remove default background color
reuse bg colors
Cosmetics
Moved reusing fills to XLSX StyleHelper
Tests and inline doc

src/Spout/Writer/Style/StyleBuilder.php

@@ -133,6 +133,19 @@ class StyleBuilder
         return $this;
     }
 
+    /**
+     *  Sets a background color
+     *
+     * @api
+     * @param string $color ARGB color (@see Color)
+     * @return StyleBuilder 
+     */
+    public function setBackgroundColor($color)
+    {
+        $this->style->setBackgroundColor($color);
+        return $this;
+    }
+
     /**
      * Returns the configured style. The style is cached and can be reused.
      *