Support reading files from standard input

It could be great to support streaming NDJSON and CSV from stdin. Streaming a file is often better when the file is smaller and compressed. For example, I have a 40M file that weighs 1GiB compressed but 15GiB uncompressed/raw.

```bash
gzcat myfile.ndjson.gz | meilisearch-importer --stdin ndjson
gzcat myfile.csv.gz | meilisearch-importer --stdin csv
```