Make the features configurable, not auto-loading

[walterheck]

Feature request

So, when you create an instance of this loader it tries to load a bunch of python packages automatically to handle certain file formats. Rather than auto-loading, I think it would make sense to have a way to say "I need PDF and google sheets support", which requires those packages and fails if you don't have them installed. That way I don't have to guess which ones I need, and why a certain file format is not being used

Motivation

See above.

Your contribution

My python skills are not very good, but happy to help review.