Add RapidJSON AutoUTFInputStream test

Adds a parsing test using RapidJSON `AutoUTFInputStream`, inspired by the thread at miloyip/rapidjson#159.

My results show (summarized):

```
Benchmarking RapidJSON (C++)
          Parse canada.json          ...  6.111 ms  351.296 MB/s
          Parse citm_catalog.json    ...  2.733 ms  602.704 MB/s
          Parse twitter.json         ...  2.023 ms  297.706 MB/s

Benchmarking RapidJSON (C++, AutoUTF)
          Parse canada.json          ... 10.171 ms  211.068 MB/s
          Parse citm_catalog.json    ...  7.745 ms  212.678 MB/s
          Parse twitter.json         ...  5.207 ms  115.663 MB/s
```

I'm not sure if this is something you want to add since you don't have variations on other libraries. However, I think it would be useful as a comparison and to use for possible future improvement of the `AutoUTF` approach.