Add a less generic subsclass of StaticItem

[benoit74]

As discussed in https://github.com/openzim/python-scraperlib/issues/109, we can benefit from creating a less generic subclass of StaticItem with more strongly typed arguments than the very generic **kwargs we have in StaticItem.

This less generic class would serve >80% of the scraper needs.

[benoit74]

@rgaudin do you think there is something left to do on this issue after #132 last changes?

I had a look at all openZIM codebase and did not find anything which is not covered.

Most scrapers are using either add_item_for or add_redirect anyway.

The only thing we forgot to perform is the modification of add_item_for to use new StaticItem parameters. But this is not really related to this issue.

[rgaudin]

I agree ; we can close this

[benoit74]

Fixed by #132