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/CSV/Writer.php

@@ -69,10 +69,11 @@ class Writer extends AbstractWriter
      *
      * @param  array $dataRow Array containing data to be written.
      *          Example $dataRow = ['data1', 1234, null, '', 'data5'];
+     * @param \Box\Spout\Writer\Style\Style $style Ignored here since CSV does not support styling.
      * @return void
      * @throws \Box\Spout\Common\Exception\IOException If unable to write data
      */
-    protected function addRowToWriter(array $dataRow)
+    protected function addRowToWriter(array $dataRow, $style)
     {
         $wasWriteSuccessful = $this->globalFunctionsHelper->fputcsv($this->filePointer, $dataRow, $this->fieldDelimiter, $this->fieldEnclosure);
         if ($wasWriteSuccessful === false) {