[TW-219] sorting by tag improved and explained

_David Patrick on 2010-12-22T13:02:17Z says:_

It's entirely possible to make a report that sorts by tag, but right now the result is a bit of a crap-shoot.
Taskwarrior will sort based on the first tag in the record, and if you know that, and it's important, you can add your tags in the desired sequence, or you can alter that sequence after-the-fact using *task ID edit*.

To improve sorting-by-tag, this behavior needs to be clearly understood and documented, and some default behaviors should be modified.

Context tags (that is any tag with a leading "@") should be the default sort-driver within a set of "normal" tags, and if there are several with (at)something, the first one wins. Reserved "special tags" should be ignored in regards to sorting, because that's not what they're for.