Incomplete zsh filter completion

The zsh completion function in `scripts/zsh/_task` seems to lack some of the available options for specifying dates as described in the man pages [Named Dates](https://taskwarrior.org/docs/named_dates/) and [Date & Time](https://taskwarrior.org/docs/dates/).
I didn't do a thorough search. Examples are `eod` and `socy`.

Also there isn't a complete list of abbreviations available in the `man` pages which I'd find very useful.

Thanks a lot for your great work to provide such a handy tool.
If you can use the help I could take a closer look at the completion function and try to contribute some additions.