Allow setup threads count + chunk size

- setup via config file (add doc + config sample)
- for transfer up/down
- chunk size can't be higher than 64Mib (hard code check)
- parallel can't be higher than 8 (hard code check)

Config idea:

```yaml
upload:
  parallel: 4  # allow false (= 1)
  chunk_size: 10485760 # 10MiB
download:
  parallel: 4 # allow false (=1)
```