Add more feature flags

[0WN463]

Adds more feature flag for a more flexible usage:

• add -d, --download-only to only download the images
• add -i, image-output to keep images instead of converting to pdf
• add -r, reader to allow user to specify their own reader

Following this changes, we no longer require all the dependencies to be needed. They are extracted and only checked if their relevant flags are toggled