allow omitting of leading xml tag (#48)

* feat(writer): allow omitting of leading xml tag

In some use-cases it is useful to group multiple svg into one larger
svg. However, in this we need to omit the <?xml tag from the child svg.

* fix: expose $standalone flag in toXMLString

Since most external applications will not instantiate SVGWriter directly,
the flag should be exposed to common methods that could use it.

src/SVGImage.php

@@ -77,11 +77,13 @@ public function __toString()
      * same XML string generated by this method, since the output is optimized
      * and formatted according to specific rules.
      *
+     * @param bool $standalone optional, false omits the leading <?xml tag
+     *
      * @return string This image's document tree as an XML string.
      */
-    public function toXMLString()
+    public function toXMLString($standalone = true)
     {
-        $writer = new SVGWriter();
+        $writer = new SVGWriter($standalone);
         $writer->writeNode($this->document);
 
         return $writer->getString();

src/Writing/SVGWriter.php

@@ -13,11 +13,13 @@
 class SVGWriter
 {
     /** @var string $outString The XML output string being written */
-    private $outString;
+    private $outString = '';
 
-    public function __construct()
+    public function __construct($isStandalone = true)
     {
-        $this->outString = '<?xml version="1.0" encoding="utf-8"?>';
+        if ($isStandalone) {
+            $this->outString = '<?xml version="1.0" encoding="utf-8"?>';
+        }
         defined('ENT_XML1') ? ENT_XML1 : define('ENT_XML1', 16);
     }