Adds support for glTF - text and binary formats (#9)

See: https://github.com/KhronosGroup/glTF
diff --git a/lib/src/default_extension_map.dart b/lib/src/default_extension_map.dart
index cfa42ed..e3ee44c 100644
--- a/lib/src/default_extension_map.dart
+++ b/lib/src/default_extension_map.dart
@@ -283,6 +283,8 @@
   'ghf': 'application/vnd.groove-help',
   'gif': 'image/gif',
   'gim': 'application/vnd.groove-identity-message',
+  'glb': 'model/gltf-binary',
+  'gltf': 'model/gltf+json',
   'gml': 'application/gml+xml',
   'gmx': 'application/vnd.gmx',
   'gnumeric': 'application/x-gnumeric',
diff --git a/lib/src/magic_number.dart b/lib/src/magic_number.dart
index 7430284..7ea10fa 100644
--- a/lib/src/magic_number.dart
+++ b/lib/src/magic_number.dart
@@ -64,5 +64,6 @@
     0xFF,
     0xFF,
     0xFF
-  ])
+  ]),
+  const MagicNumber('model/gltf-binary', const [0x46, 0x54, 0x6C, 0x67]),
 ];