Capitalized R detection

Seems like a simple bug:

```
$ sloc ./*.R
C:\Users\Birke\AppData\Roaming\npm\node_modules\sloc\lib\sloc.js:195
throw new TypeError("File extension '" + lang + "' is not supported");
^

TypeError: File extension 'R' is not supported
at getCommentExpressions (C:\Users\Birke\AppData\Roaming\npm\node_modules\sloc\lib\sloc.js:195:17)
at slocModule (C:\Users\Birke\AppData\Roaming\npm\node_modules\sloc\lib\sloc.js:376:31)
at C:\Users\Birke\AppData\Roaming\npm\node_modules\sloc\lib\cli.js:84:19
at tryToString (fs.js:456:3)
at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:443:12)

$ sloc ./*.R -a R=r

---------- Result ------------
...
```