Feature: Camel case from/to underscore conversion

Camel case to underscore.
- `SomeWords` => `some_words`
- `HTTPServer` => `http_server`
- `NoHTTPS` => `no_https`

Underscore to camel case.
- `some_words` => `SomeWords`
- `http_server` => `HttpServer`
- `no_https` => `NoHttps`
- `_complex__case_` => `_Complex_Case_`
