Add .odd to XML extensions

See discussion at #2238. 

[ODD](http://en.wikipedia.org/wiki/Text_Encoding_Initiative#ODD) is an XML-based vocabulary for defining schemas for Text Encoding Initiative (TEI) projects, used widely in the humanities.  ODD should be rendered using XML syntax but is currently displayed as plain text (see [this example](https://github.com/wolfgangmm/tei-simple-pm/blob/master/odd/teisimple.odd)).  There are [nearly 500 ODD files](https://github.com/search?q=extension%3Aodd+NOT+djfhdirijdskqdhd&type=Code&utf8=%E2%9C%93) in GitHub, and this number is projected to grow.

lib/linguist/languages.yml

@@ -3326,6 +3326,7 @@ XML:
   - .mxml
   - .nproj
   - .nuspec
+  - .odd
   - .osm
   - .plist
   - .pluginspec