Internal improvements: better modelling of "sequential/chronological" sort-tags This release formalizes the concepts of `sequential sort-tags` vs. `chronological sort-tags`: 1. A _sort-tag_ is composed of a number of counters, which can be numerical, e.g. `123.28` or combined numerical/letter based, e.g. `123ab`. 2. A counter is set of digits (base 10) `123` or a set of lowercase letters (base 26) `ab`. 3. A letter based counter can be maximal 2 letters wide. Its maximum is `zz` (cf. `filename.sort_tag.letters_in_succession_max`). 4. A _sequential sort-tag_ is a sort-tag that whose counters are at most 3 digits wide (cf. `sort_tag.sequential.digits_in_succession_max`). 5. The filter `incr_sort_tag` increments only sequential sort-tags. 6. In order not to confuse sequential and chronological sort-tags, it is recommended to always write out the year in chronological sort-tags with 4 digits, e.g. `2013-08-10` or `20130810`.