nginx: add lua module

This adds the `ngx_lua` module as an optional component.

I named the argument key `ngx_lua` since `lua` is already taken, though it now does not match the other modules' argument keys.  ([Upstream](https://github.com/openresty/lua-nginx-module) refers to the package by various names, `ngx_lua` being one of them).

I used `luajit` since that's all I've tested with (on darwin and linux), but it's certainly possible to use with plain lua. Not quite sure on the best way to add support for both, should that be desired.

After building, the output of `nginx -V` looks like this:

```
nginx version: nginx/1.6.1
...
--add-module=/nix/store/aaa-git-export --add-module=/nix/store/bbb-git-export --add-module=/nix/store/ccc-git-export
...
```

It's a minor annoyance that all the modules from git end in `git-export` rather than some identifying string. Is there any way around that?