eleventyConfig.addExtension is too case-sensitive

If I do
```javascript
eleventyConfig.addExtension("vZome", {...})
```
I do NOT get processing of files with the extension `.vZome`.  It only processes the files if the argument to `addExtension` is all lowercase. 

This is in contrast with `eleventyConfig.addTemplateFormats`, which is unaffected by the case of the argument.
