add image receive mode

[gilcu3]

This PR adds support for selecting how the bot send images generated with DALL·E though an env variable IMAGE_RECEIVE_MODE. Currently, the bot sends images as photos, which are compressed by telegram. By allowing sending images as documents, we receive the full resolution image in png format. I left the photo as the default option for backward compatibility, although I think document is always better :).

One possible addition, once DALL·E 3 is merged, is to respond with the revised_prompt (from the json response) as a caption for the photo. It seems DALL·E 2 responses do not include this.

[n3d1117]

Thanks @gilcu3, I like this (just fixed a little name mismatch for the env variable, there was a IMAGE_RECEIVE_MODE instead of IMAGE_FORMAT).

I'm going to merge this, feel free to open a new PR if you find the time to work on the captions for DALL·E!