Use Dutch language code nl

[cocoatomo]

This PR resolves issue #43 .

• Replace du with nl contained in filenames and file contents
• Leave lunr.du.js file as an alias of lunr.du.js

[MihaiValentin]

Thanks for this contribution and also for considering preventing breaking changes!

The lunr.<locale> and min/lunr.<locale> files are generated at build time, running npm run build.
Modifying them directly will just override them on next build.

I think the best way for now is to duplicate (create a new entry in build.js with nl, but identical to the one with du, and run the build script. You could also change the build script, so it could inject a message in the du file, like console.warning(message), in which case the message for du would be "The du language code was corrected to nl. Please use the nl, as in the next versions the du file will be removed".

Then, whenever I'll build a new breaking change version of the npm package, I'll delete the du file.

What do you think?

[MihaiValentin]

Added NL and deprecated DU (added a console.warn message). I'll close this PR.

[cocoatomo]

Oh, thank you for your proposal and works 👍