Group locations in same timezone

First of all, thx for this neat project, it's very helpful for me!

When specifying multiple locations in same timezone (i.e. Europe/London and UTC), right now only UTC will get displayed.

» undertime Los_Angeles EST Bogota New_York Sao_Paulo UTC London Europe/Berlin
WARNING:root:skipping zone America/Bogota with existing offset -5:00:00
WARNING:root:skipping zone America/New_York with existing offset -5:00:00
WARNING:root:skipping zone Europe/London with existing offset 0:00:00
WARNING:root:skipping zone Europe/Berlin with existing offset 1:00:00
┌───────┬─────────────────────┬───────┬───────────────────┬───────┐
│  CET  │ America/Los_Angeles │  EST  │ America/Sao_Paulo │  UTC  │
├───────┼─────────────────────┼───────┼───────────────────┼───────┤
│ 00:00 │        15:00        │ 18:00 │       20:00       │ 23:00 │

It would be nice if all given locations from the same timezone were grouped and displayed in the title of the colum, like:

» undertime Los_Angeles EST Bogota New_York Sao_Paulo UTC London Europe/Berlin
┌───────┬─────────────────────┬───────┬───────────────────┬───────┐
│  CET  │ America/Los_Angeles │  EST  │ America/Sao_Paulo │  UTC  │
│       │                     │       │                   │London │
├───────┼─────────────────────┼───────┼───────────────────┼───────┤
│ 00:00 │        15:00        │ 18:00 │       20:00       │ 23:00 │