Add option to sort base on certain locale.

[SirZenith]

This pull request adds implementation for sorting with user provided locale (Fixes #323).

An new option locale is introduced, which has default value ''. And this option supports setlocal.

• When this option is set to empty string, localized sorting is disabled, and program should behave the same as before.
• When set to certain IETF BCP 47 language tag, file list will be sort with cultural habits of specified language.
• Special value * is used to indicate reading locale value from environment.

[joelim-work]

Hi, thanks for your contribution.

I think it is a good idea to support locale-aware sorting, but I would prefer to implement it in a way that doesn't introduce too much complexity. I have left some comments which you might want to look at, although I personally don't have much experience with using the collate library.

Also note that there is a change freeze for a couple of weeks while I am planning to make a new release, so this PR will have to wait until after that point to be merged.

[joelim-work]

Thanks for clarifying your expectations about how locale should work. I've gone through the code in a bit more detail this time, but this should probably be the last set of comments and I think the PR looks much better overall.

Also FYI you can add something like Fixes #323 to the PR description, which will link the issue properly and automatically close it when the PR is merged.

[joelim-work]

Thanks, I think it looks good now. I have given my approval, but will hold off merging it until after the freeze period is over.