feat(#104): add options to delete files before/after upload

[rlmestre]

Fixes #104. Added the following options to delete files as part of an upload:

• deleteAfterUpload: After the upload is successful, get a list of files in the bucket subfolder, exclude newly uploaded files, and delete all remaining objects.
• deleteBeforeUpload Before uploading, gets a list of files in the bucket subfolder and deletes them all.

I'm not sure if it's worth it to maintain two options, but since both were brought up in the issue, I'm opening this as a draft for discussion.

EDIT: Fixes applied, tested in several scenarios. Should be safe to try now.

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[Jefiozie]

I'm going to merge this in and see if I can get a test around it. Without it think it is a bit tricky to release it. Thanks for your work @rlmestre