Extract main to update

Extracted most of the functionality from main into a new update fn. This is to allow room for command flags.

src/main.rs

@@ -21,6 +21,10 @@ mod wrappers;
 fn main() {
     println!("{}", copyright_notice());
 
+    update();
+}
+
+fn update() {
     if let Err(e) = pacman::check() {
         error_println(e.to_string());
         return;