refactor: Allow starship to be better used programmatically (#509)

Structure the prompt as a function that returns a string instead of writing directly to stdout.

This makes it easier to embed Starship into other Rust programs such as shells written in Rust. It also decouples the arguments from the context for more programmatic initialization of the context.

src/print.rs

@@ -1,5 +1,6 @@
 use clap::ArgMatches;
 use rayon::prelude::*;
+use std::fmt::Write as FmtWrite;
 use std::io::{self, Write};
 
 use crate::context::Context;
@@ -9,14 +10,18 @@ use crate::modules;
 
 pub fn prompt(args: ArgMatches) {
     let context = Context::new(args);
-    let config = context.config.get_root_config();
-
     let stdout = io::stdout();
     let mut handle = stdout.lock();
+    write!(handle, "{}", get_prompt(context)).unwrap();
+}
+
+pub fn get_prompt(context: Context) -> String {
+    let config = context.config.get_root_config();
+    let mut buf = String::new();
 
     // Write a new line before the prompt
     if config.add_newline {
-        writeln!(handle).unwrap();
+        writeln!(buf).unwrap();
     }
 
     let mut prompt_order: Vec<&str> = Vec::new();
@@ -48,13 +53,15 @@ pub fn prompt(args: ArgMatches) {
         // Skip printing the prefix of a module after the line_break
         if print_without_prefix {
             let module_without_prefix = module.to_string_without_prefix();
-            write!(handle, "{}", module_without_prefix).unwrap()
+            write!(buf, "{}", module_without_prefix).unwrap()
         } else {
-            write!(handle, "{}", module).unwrap();
+            write!(buf, "{}", module).unwrap();
         }
 
         print_without_prefix = module.get_name() == "line_break"
     }
+
+    buf
 }
 
 pub fn module(module_name: &str, args: ArgMatches) {