Add support to choose default package size and delivery instructions on marktplaats

[spo0nman]

The marktplaats site has changed a bit so fixes for:
    - choosing a default delivery method
    - choosing a default package size
Some changess to make the script more easy to use:
    - Better installation instructions for mac.
    - Ability to change timeouts and other delays
    - Ability to upload only to one site

[spo0nman]

Hello @jort-dev thanks for writing this. I had about a 100 books to sell and I was able to use your script to effectively upload to the marktplaats site.

The site has changed slightly in the past few months and the default shipping and package selection was not working so my fix fixes that.

I also noticed a few issues while using it. sometimes the photo uploads take longer than expected and confounds selenium so I added ability to change the delays.

Thanks again. Hope this helps.