feat: add --monday-first arg

[zed]

[aaossa]

This one is a beautiful feature. My only comment is that it looks a bit weird to display "Mon", "Wed", and "Fri" when Monday is the first day (the last two rows do not display their corresponding days, see the figure). If the flag is enabled, maybe it should display "Tue", "Thu", and "Sat"... but I think that I don't really have a strong opinion on this. Any ideas?

Also, I just noticed that if the flag is enabled, the key variable in the last loop does need a + 1 to adjust the alignment of the displayed days. This fix might break if it's Sunday, because the script might create a new column when is not really needed (hard to test btw...)

(This one was patched using the + 1 fixup)