Add support for padded range expansion.

Created by: ProgVal

Problem: I wanted {03..07} to expand to 03 04 05 06, as on bash or zsh.

Solution: I added a new argument nb_digits to range-generating functions, so the output number is padded to get the expected number of digits. Note that - counts as a digits in order to get proper alignment when dealing with both negative and positive numbers (like bash and zsh do).

Changes introduced by this pull request:

• The behavior of ranges like {03..07} changes.

Drawbacks: slightly slower, because we have to do some lookup (of at most two characters) and format! uses a loop to pad

TODOs: nothing

Fixes: my personal workflow when I want to match files numbered with leading zeroes to sort alphabetically

State: ready

Blocking/related: none

Other: none