Eliminating `prettytable-rs` deps

[Myriad-Dreamin]

First, thanks for providing the great iso crates. I value to minimalize dependencies in Cargo.lock, and track the last uses of lazy_static, so my report following isn't a bug but a minor request. The dependency chain to eliminate is following:

rust_iso3166 and rust_iso639 depend on prettytable-rs
prettytable-rs depends on lazy_static

I note that it is only used in the main.rs. Could we eliminate it?

[zhangjiayin]

hi @Myriad-Dreamin Thank you for the thoughtful suggestion!
Feel Free to submit a PR Please.