[TI-36] Hidden and/or non-sorting tags

_Paul Kishimoto on 2016-08-19T21:17:02Z says:_

Also related to [this extension|https://gist.github.com/khaeru/860b60a4e42420fe35f70b7a98294f6e], Toggl reports a unique ID for every entry. At first, I had the extension write these as a tag like `toggl_id:ID`. But I found this prevented Timewarrior from grouping these intervals together for reporting.

I also use a variation on the [`on-modify.timewarrior` hook|https://git.tasktools.org/projects/TM/repos/timew/browse/ext/on-modify.timewarrior] for Taskwarrior that adds a tag like `uuid:` to Timewarrior intervals. In this case, the UUID is useful for later matching, especially if the task description is changed in Taskwarrior; but it would be useful to be able to _omit_ it from Timewarrior displays.

Perhaps:
* Tags beginning with `_`, `.` or some other special character could be hidden and/or ignored when reporting, and/or
* A configuration option like `tag..ignore = yes` could be added.