Feature request: sentence case

[Nick-Mazuk]

Hi there, thanks for the crate!

Quick feature request: sentence case. Here, every character is lowercase except the first character which is uppercase.

use convert_case::{Case, Casing};

assert_eq!("My variable name", "My variable NAME".to_case(Case::Sentence));

This could easily be implemented by reusing the lower case functionality.

Since this is easy to create, this is a low priority feature request. Feel free to close this request if you think it's not worth adding it. Still I think it's more ergonomic to include it directly in the library.