Brace expansion: zero-padding and capital letters not working #1042

Hello,

First of all, thanks for the truly excellent library!

I noticed a couple of brace expansions that don't work with this library, but do work in bash, and wanted to flag them here, since I find them quite useful and believe others may as well:

1. Sequence expressions don't allow zero-padding
2. Sequence expressions don't work with capital letters

Examples:

Here's what's described in bash(1)/Brace Expansion:

[...]

A sequence expression takes the form {x..y[..incr]}, where x and y are either integers or letters, and incr, an optional increment, is an integer. When integers are supplied, the expression expands to each number between x and y, inclusive. Supplied integers may be prefixed with ‘0’ to force each term to have the same width. When either x or y begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary. When letters are supplied, the expression expands to each character lexicographically between x and y, inclusive, using the default C locale. Note that both x and y must be of the same type (integer or letter). When the increment is supplied, it is used as the difference between each term. The default increment is 1 or -1 as appropriate.