More generic interval for percentage calculations

[t4nec0]

First of all, thanks for this extension, it's great.

I'd need to use it on more generic intervals rather than :today or :month like, for example, a month in the past.
When I try now I get a subtle error with tags:

$ tw percentage 2020-01-01 - 2020-12-31
This report can only report one tag per session.