Open problem pages in the reverse order

[estshorter]

Currently, cargo compete open open pages in ascending order of difficulty (A to F in AtCoder), but I think this should be done in the reverse order. This is because the most difficult problem is shown in a browser after executing the command.

possibly related code: https://github.com/qryxip/cargo-compete/blob/8a148d94f61d6bc39bf65ce995476386546ec633/src/open.rs#L17-L21