[XML] Add support for dtd/ent/mod files (sublimehq#2152)

This commit exposes the doctype definition part of the XML.sublime-syntax to be used by dedicated DTD files.

Notes:

1. The test cases for doctype definitions are located in the
   syntax_test_xml.xml.
2. This file type seems not to be used by any officially listed 3rd
   party package at packagecontrol.io so far.

XML/DTD.sublime-syntax

@@ -0,0 +1,12 @@
+%YAML 1.2
+---
+name: DTD
+file_extensions:
+  - dtd
+  - ent
+  - mod
+scope: text.xml.dtd
+
+contexts:
+  main:
+    - include: Packages/XML/XML.sublime-syntax#dtd