Add a way to select each day.

src/main.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::io;
 
 fn get_calibration_digits() -> u32 {
@@ -98,5 +99,14 @@ fn get_calibration_digits_alnum() -> u32 {
 }
 
 fn main() {
-  println!("Sum: {}", get_calibration_digits_alnum());
+  let args: Vec<String> = env::args().collect();
+  if args.len() < 2 {
+    println!("Select a day (i.e. day1a, day1b)");
+    return;
+  }
+  match args[1].as_str() {
+    "day1a" => println!("Sum: {}", get_calibration_digits()),
+    "day1b" => println!("Sum: {}", get_calibration_digits_alnum()),
+    _ => println!("Unrecognized day: {}", args[1]),
+  }
 }