Add the `--config-dir` option
Add the `--config-dir` option

With `--config-dir` option, it allows the configuration to be seperated
into multiple files.
In the case if you don't want to put all query in the same configuration
file, it would support:

```
├── default.cfg
└── config
    ├── query1.cfg
    ├── query2.cfg
    └── query3.cfg
```

Braedon/prometheus-es-exporter