Playground infraestructure (tests, etc) additions. (#3179)

* Playground infraestructure (tests, etc) additions.

A few things to note:

* Nu can be started with a custom configuration file (`nu --config-file /path/to/sample_config.toml`). Useful for mocking the configuration on test runs.
* When given a custom configuration file Nu will save any changes to the file supplied appropiately.
* The `$nu.config-path` variable either shows the default configuration file (or the custom one, if given)
* We can now run end to end tests with finer grained control (currently, since this is baseline work, standard out) This will allow to check things like exit status, assert the contents with a format, etc)

* Remove (for another PR)

src/main.rs

@@ -1,14 +1,21 @@
 use clap::{App, Arg};
 use log::LevelFilter;
-use nu_cli::create_default_context;
+use nu_cli::{create_default_context, NuScript, Options};
 use nu_command::utils::test_bins as binaries;
 use std::error::Error;
-use std::fs::File;
-use std::io::prelude::*;
 
 fn main() -> Result<(), Box<dyn Error>> {
+    let mut options = Options::new();
+
     let matches = App::new("nushell")
         .version(clap::crate_version!())
+        .arg(
+            Arg::with_name("config-file")
+                .long("config-file")
+                .help("custom configuration source file")
+                .hidden(true)
+                .takes_value(true),
+        )
         .arg(
             Arg::with_name("loglevel")
                 .short("l")
@@ -84,6 +91,11 @@ fn main() -> Result<(), Box<dyn Error>> {
         return Ok(());
     }
 
+    options.config = matches
+        .value_of("config-file")
+        .map(std::ffi::OsString::from);
+    options.stdin = matches.is_present("stdin");
+
     let loglevel = match matches.value_of("loglevel") {
         None => LevelFilter::Warn,
         Some("error") => LevelFilter::Error,
@@ -125,28 +137,20 @@ fn main() -> Result<(), Box<dyn Error>> {
     match matches.values_of("commands") {
         None => {}
         Some(values) => {
-            let script_text: String = values
-                .map(|x| x.to_string())
-                .collect::<Vec<String>>()
-                .join("\n");
-            futures::executor::block_on(nu_cli::run_script_file(
-                script_text,
-                matches.is_present("stdin"),
-            ))?;
+            options.scripts = vec![NuScript::code(values)?];
+
+            futures::executor::block_on(nu_cli::run_script_file(options))?;
             return Ok(());
         }
     }
 
     match matches.value_of("script") {
-        Some(script) => {
-            let mut file = File::open(script)?;
-            let mut buffer = String::new();
-            file.read_to_string(&mut buffer)?;
+        Some(filepath) => {
+            let filepath = std::ffi::OsString::from(filepath);
 
-            futures::executor::block_on(nu_cli::run_script_file(
-                buffer,
-                matches.is_present("stdin"),
-            ))?;
+            options.scripts = vec![NuScript::source_file(filepath.as_os_str())?];
+
+            futures::executor::block_on(nu_cli::run_script_file(options))?;
             return Ok(());
         }
 
@@ -159,7 +163,7 @@ fn main() -> Result<(), Box<dyn Error>> {
 
             #[cfg(feature = "rustyline-support")]
             {
-                futures::executor::block_on(nu_cli::cli(context))?;
+                futures::executor::block_on(nu_cli::cli(context, options))?;
             }
 
             #[cfg(not(feature = "rustyline-support"))]