Feature: specify AWS region in config

[whlteXbread]

Hi, I was taking QSL for a spin and ran into some issues when accessing objects in a bucket outside my default region.

It might be nice to specify the region in the config—that way I could segment my data by region and then run multiple labeling sessions sequentially.

There might be a smarter way to do this, but from my quick look through the codebase it looks like checking the config could happen right about here and not be too intrusive: https://github.com/faustomorales/qsl/blob/b159cf99b891fb6477de1f58a3c7eadccc8e0083/qsl/files.py#L90

Thoughts?

[faustomorales]

Great idea! I don't really have the bandwidth to work on this right now but a PR would be welcome!