WIP Selective gzip instead of compressing everything

Currently, if you enable the `--gzip` flag, this will gzip **everything**. But generally, you only want to gzip text content.

This change follows the same patterns I've found in django-extensions https://github.com/django-extensions/django-extensions/blob/master/django_extensions/management/commands/sync_s3.py#L351-L354 and django-storages https://github.com/django-extensions/django-extensions/blob/master/django_extensions/management/commands/sync_s3.py#L351-L354

#### TODO
* [ ] have a way to gzip everything like before this PR was opened
* [ ] have a way to control which content types get gzipped