Support excluding specific hours of a day for a specific day

Right now we are able to exclude:
1. whole days with `timew config exclusions.days.2021_12_24 = off`
2. ranges within a day of a week with `timew config exclusions.monday '<9:00 12:40-13:10 >17:30'

But there is no way to exclude part of a single day, e.g. the afternoon of the 24th of December.
Intuitively I would expect something like `timew config exclusions.days.2021_12_24 = '>12:00'` to work.

Thanks for this simple yet great tool :)

No files or tests are named. Start by tracing the existing day-based and weekday exclusion configuration handling, then determine where date-specific time ranges are parsed and applied. Done means a configuration such as exclusions.days.2021_12_24 = '>12:00' excludes only that period while preserving whole-day and weekday exclusions.

Written by the indexing model from the issue text.