feat: add support for nu shell (#2847)

src/main.rs

@@ -41,7 +41,7 @@ fn main() {
     let shell_arg = Arg::with_name("shell")
         .value_name("SHELL")
         .help(
-            "The name of the currently running shell\nCurrently supported options: bash, zsh, fish, powershell, ion, elvish, tcsh",
+            "The name of the currently running shell\nCurrently supported options: bash, zsh, fish, powershell, ion, elvish, tcsh, nu",
         )
         .required(true);