Added support for mergeCells, cell height, shrink to fit

Added support for
    mergeCells:
        // mergeCells (B2:G2), you may use CellHelper::getColumnLettersFromColumnIndex() to convert from "B2" to "[1,2]"
	    $writer->mergeCells([1,2], [6, 2]);

    cell height:
        $row->setHeight(30);

    shouldShrinkToFit:
        $style->setShouldShrinkToFit();

These changes are implemented for XLSX as that's what I need and test spout on.

src/Spout/Writer/XLSX/Manager/Style/StyleManager.php

@@ -258,6 +258,10 @@ EOD;
                 if ($style->shouldWrapText()) {
                     $content .= ' wrapText="1"';
                 }
+                if ($style->shouldShrinkToFit()) {
+                    $content .= ' shrinkToFit="true"';
+                }
+
                 $content .= '/>';
                 $content .= '</xf>';
             } else {