refactor(cli): revert .subcommand_required(true)

This commit reverts the flag added in #86
which prevents the binary from being
invoked without a subcommand. It turns out
that `mdBook` calls the binary without args
to invoke the preprocessor and it is not
worth detecting if the parent process was
mdbook or not.

src/bin/mdbook-catppuccin.rs

@@ -47,7 +47,6 @@ pub fn make_app() -> Command {
         .about(crate_description!())
         .author(crate_authors!())
         .version(crate_version!())
-        .subcommand_required(true)
         .subcommand(
             command!("supports")
                 .arg(