add `id` attribute to Task

[domm]

Currently a lot of logic is based on the first tag of a task. Various plugins (the ones implementing issue trackers) try to put the current issue/card/ticket name at the start of the tag list, but this is not very stable (esp because this is not stable due to module loading order).

So I propose to "upgrade" what's currently in the first tag to the "id" of the task, i.e. the issue/ticket/card number/ident, or (if you're not using any issue tracker) some other identifier of the task.

This will allow users to report / calc data based on distinct tasks, even if they did not handle the task in one go.

The various Plugins should automatically set id based on the issue id:

tracker start --rt 123 will do all the RT handling, but also set id to RT123.

tracker start --rt 123 --id "Rev the Flux Compensator" would override that, though I have no idea why one would do that.

tracker start --id "finetune-spec" --tag meeting makes a bit more sense, esp for project where you don't have an issue tracker

Using the new prefixes & various Plugins this could look like:

tracker start --trello 123-rev-flux-compensator --cat feature --billing A2021-01
Started working on examle-project 123-rev-flux-compensator (.trello=123-rev-flux-compensator, .cat=feature, .billing=A20201-01) Rev the Flux Compensator at 19:23:23

(edit: s/name/id/g)