Add parsing penwidth attribute

'penwidth' defines the line width as 'width'. If both are exist,
'penwidth' is used and 'width' is discarded as described in [1].
This update fixes #24.

[1] https://www.graphviz.org/doc/info/attrs.html#d:penwidth

lib/network/dotparser.js

@@ -1026,7 +1026,25 @@ function parseAttributeList() {
 
   attr_list = parseDirAttribute(attr_names, attr_list);
 
-  var nof_attr_list = attr_list.length;
+  // parse 'penwidth'
+  var nof_attr_list;
+  if (attr_names.includes('penwidth')) {
+    var tmp_attr_list = [];
+
+    nof_attr_list = attr_list.length;
+    for (i = 0; i < nof_attr_list; i++) {
+      // exclude 'width' from attr_list if 'penwidth' exists
+      if (attr_list[i].name !== 'width') {
+        if (attr_list[i].name === 'penwidth') {
+          attr_list[i].name = 'width';
+        }
+        tmp_attr_list.push(attr_list[i]);
+      }
+    }
+    attr_list = tmp_attr_list;
+  }
+
+  nof_attr_list = attr_list.length;
   for (i = 0; i < nof_attr_list; i++) {
     setValue(attr_list[i].attr, attr_list[i].name, attr_list[i].value)
   }