Possibility to set filename of literal data packet (tag 11)

The filename of the literal data packet was hardcoded to msg.txt. Now
one has the possibility to manually set the filename of the literal
data packet.

src/packet/literal.js

@@ -38,6 +38,7 @@ function Literal() {
   this.format = 'utf8'; // default format for literal data packets
   this.data = ''; // literal data representation as native JavaScript string or bytes
   this.date = new Date();
+  this.filename = 'msg.txt';
 }
 
 /**
@@ -84,6 +85,24 @@ Literal.prototype.getBytes = function () {
 };
 
 
+/**
+ * Sets the filename of the literal packet data
+ * @param {String} filename Any native javascript string
+ */
+Literal.prototype.setFilename = function (filename) {
+  this.filename = filename;
+};
+
+
+/**
+ * Get the filename of the literal packet data
+ * @returns {String} filename 
+ */
+Literal.prototype.getFilename = function() {
+  return this.filename;
+};
+
+
 /**
  * Parsing function for a literal data packet (tag 11).
  *
@@ -117,7 +136,7 @@ Literal.prototype.read = function (bytes) {
  * @return {String} string-representation of the packet
  */
 Literal.prototype.write = function () {
-  var filename = util.encode_utf8("msg.txt");
+  var filename = util.encode_utf8(this.filename);
 
   var data = this.getBytes();