feat(config): add optional filename attribute

[Teufelchen1]

Hi 🦌

This adds a filename attribute to the config. It provides one possible solution for #179

Downsides of my approach:

• The user must take care of providing a name that conforms with the filesystem in use
• Related, there is no sensitization at all, the filename attribute could be used in a binary planting attack or similar (unlikely)
• I included the filename in the duplication check, this enables to actually have duplicated sites, once by the default name and once by the user provided name. (I thought this to be a good price to pay in order to detect duplicated user provided names)

[EdJoPaTo]

The only downside I currently see is the single filename for multiple urls. Not sure how to handle that better. Currently it fails on config validation as the filename exists multiple times.

- url:
    - https://edjopato.de/
    - https://edjopato.de/post/
  filename: bla
  editors:
    …

Guess I'll just allow that and continue on.

[EdJoPaTo]

Thank you for your effort on this @Teufelchen1!