refactor: pull things into own files, have a library

src/cli.rs

@@ -0,0 +1,18 @@
+pub fn read_input() -> Option<String> {
+    use std::io::{BufRead, Write};
+
+    print!("osdb > ");
+    std::io::stdout().flush().expect("failed to flush stdout");
+
+    let mut input = String::new();
+    let len = std::io::stdin()
+        .lock()
+        .read_line(&mut input)
+        .expect("failed to read input from stdin");
+
+    if len == 0 {
+        None
+    } else {
+        Some(input)
+    }
+}