Allow external binary to register custom commands. (#1780)

This changeset contains everything that a separate binary needs to
register its own commands (including the new help function). It is
very possible that this commit misses other pub use exports, but
the contained ones work for our use cases so far.

src/main.rs

@@ -1,5 +1,6 @@
 use clap::{App, Arg};
 use log::LevelFilter;
+use nu_cli::{create_default_context, EnvironmentSyncer};
 use std::error::Error;
 use std::fs::File;
 use std::io::{prelude::*, BufReader};
@@ -128,7 +129,10 @@ fn main() -> Result<(), Box<dyn Error>> {
                 "Welcome to Nushell {} (type 'help' for more info)",
                 clap::crate_version!()
             );
-            futures::executor::block_on(nu_cli::cli())?;
+
+            let mut syncer = EnvironmentSyncer::new();
+            let context = create_default_context(&mut syncer, true)?;
+            futures::executor::block_on(nu_cli::cli(syncer, context))?;
         }
     }