Implement day 4.

Nice code-reuse between 1 and 2. I'm not accustomed to this.

2018/src/main.rs

@@ -1,4 +1,5 @@
 extern crate clap;
+extern crate chrono;
 extern crate regex;
 use clap::{Arg, App};
 use std::fs;
@@ -8,12 +9,14 @@ pub mod common;
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;
 
 fn get_impl(day: &str) -> Box<common::Solution> {
     match day.parse() {
         Ok(1) => Box::new(day01::Day01::new()),
         Ok(2) => Box::new(day02::Day02::new()),
         Ok(3) => Box::new(day03::Day03::new()),
+        Ok(4) => Box::new(day04::Day04::new()),
         Ok(val) => panic!("Unimplemented day {}", val),
         _ => panic!("Invalid number"),
     }
@@ -59,7 +62,7 @@ mod tests {
     #[test]
     fn test_get_impl() {
         // Verify that we can load all days
-        let last_implemented = 2;
+        let last_implemented = 4;
         for d in 1..=last_implemented {
             get_impl(&format!("{}", d));
         }