Add support for styling

Added top level methods on the Writer:
 - addRowWithStyle()
 - addRowsWithStyle()

Added a style builder, to easily create new styles.
Each writer can specify its own default style and all styles will automatically inherit from it.

For now, the style properties supported are:
 - bold
 - italic
 - underline
 - strikethrough
 - font size
 - font name
 - wrap text (alignment)

src/Spout/Writer/Style/StyleBuilder.php

@@ -0,0 +1,113 @@
+<?php
+
+namespace Box\Spout\Writer\Style;
+
+/**
+ * Class StyleBuilder
+ * Builder to create new styles
+ *
+ * @package Box\Spout\Writer\Style
+ */
+class StyleBuilder
+{
+    /** @var Style Style to be created */
+    protected $style;
+
+    /**
+     *
+     */
+    public function __construct()
+    {
+        $this->style = new Style();
+    }
+
+    /**
+     * Makes the font bold.
+     *
+     * @return StyleBuilder
+     */
+    public function setFontBold()
+    {
+        $this->style->setFontBold();
+        return $this;
+    }
+
+    /**
+     * Makes the font italic.
+     *
+     * @return StyleBuilder
+     */
+    public function setFontItalic()
+    {
+        $this->style->setFontItalic();
+        return $this;
+    }
+
+    /**
+     * Makes the font underlined.
+     *
+     * @return StyleBuilder
+     */
+    public function setFontUnderline()
+    {
+        $this->style->setFontUnderline();
+        return $this;
+    }
+
+    /**
+     * Makes the font struck through.
+     *
+     * @return StyleBuilder
+     */
+    public function setFontStrikeThrough()
+    {
+        $this->style->setFontStrikeThrough();
+        return $this;
+    }
+
+    /**
+     * Sets the font size.
+     *
+     * @param int $fontSize Font size, in pixels
+     * @return StyleBuilder
+     */
+    public function setFontSize($fontSize)
+    {
+        $this->style->setFontSize($fontSize);
+        return $this;
+    }
+
+    /**
+     * Sets the font name.
+     *
+     * @param string $fontName Name of the font to use
+     * @return StyleBuilder
+     */
+    public function setFontName($fontName)
+    {
+        $this->style->setFontName($fontName);
+        return $this;
+    }
+
+    /**
+     * Makes the text wrap in the cell if it's too long or
+     * on multiple lines.
+     *
+     * @return StyleBuilder
+     */
+    public function setShouldWrapText()
+    {
+        $this->style->setShouldWrapText();
+        return $this;
+    }
+
+    /**
+     * Returns the configured style. The style is cached and can be reused.
+     *
+     * @return Style
+     */
+    public function build()
+    {
+        return $this->style;
+    }
+}