New servlet to expose the language tokens of the PlantUML engine

Same as `java -jar plantum.jar -language`, a new servlet is added to
expose the tokens supported by the current version of the PlantUML engine.
Useful to support tools relying on a running PlantUML server to support
syntax highlight.

src/main/java/net/sourceforge/plantuml/servlet/LanguageServlet.java

@@ -0,0 +1,54 @@
+/* ========================================================================
+ * PlantUML : a free UML diagram generator
+ * ========================================================================
+ *
+ * Project Info:  http://plantuml.sourceforge.net
+ *
+ * This file is part of PlantUML.
+ *
+ * PlantUML is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * PlantUML distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
+ * License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
+ * USA.
+ */
+package net.sourceforge.plantuml.servlet;
+
+import net.sourceforge.plantuml.syntax.LanguageDescriptor;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+import java.io.PrintStream;
+
+/**
+ * Servlet used to inspect the language keywords of the running PlantUML server.
+ * Same as {@code java -jar plantuml.jar -language}
+ */
+public class LanguageServlet extends HttpServlet {
+
+    @Override
+    protected void doPost(HttpServletRequest request, HttpServletResponse response)
+            throws ServletException, IOException {
+        throw new ServletException(new UnsupportedOperationException("The Language servlet only handles GET requests"));
+    }
+
+    @Override
+    protected void doGet(HttpServletRequest request, HttpServletResponse response)
+            throws ServletException, IOException {
+        final PrintStream ps = new PrintStream(response.getOutputStream());
+        response.setContentType("text/text");
+        new LanguageDescriptor().print(ps);
+    }
+}