Hardcoded file extensions

[SudoCat]

Right now, we have the "accepted" file extensions hard coded into the loader.

https://github.com/SudoCat/Nunjucks-Isomorphic-Loader/blob/master/src/node-loader.js#L54

We could instead provide a configuration option to allow users to customise this.

Even cooler would be if we could detect the "test" which has been applied to the loader, and then use this to match the extensions. This would remove the amount of user configuration required, however that could provide more unreliable.