Support for other (non ISO) week definitions

[bcourtine]

This feature has already been asked in comments to issue #139 .

For example, in my French theater company, we use "theater weeks". Theses weeks are based on theater movie releases, that occur on Wednesday.

Since we needed it, I just released a chrono_ext crate with support for custom weeks ("theater weeks", sunday based weeks, and more generally any user defined week system). Source is available here: https://github.com/bcourtine/chrono-ext

I don't know if enough users are interested to justify an integration in chrono, but if you agree, you can of course integrate that code (but it probably requires some cleaning and performance improvements).

[quodlibetor]

For now I'd definitely be happy to take a PR that links to that repo in the readme! And yeah I agree that it's a reasonable thing for chrono in general.

[bcourtine]

Hello,

Thanks for the quick reply. I will do that readme PR.

To create a code PR, some work is probably needed to harmonize (and optimize) the code… and I don't know how to do that cleanly. But if you have some clues, I can try to work on it.