Ctrl+C handling (#75)

As stated [here](https://doc.rust-lang.org/std/io/trait.BufRead.html#errors-1), `read_until` (and `read_line`) ignore Ctrl+C, so Topgrade does not respond to Ctrl+C in the retry prompt, and instead will exit only when enter is pressed after Ctrl+C. This is undesirable, so this pull request is a WIP until we find a solution.

src/ctrlc/mod.rs

@@ -0,0 +1,9 @@
+#[cfg(unix)]
+mod unix;
+#[cfg(unix)]
+pub use self::unix::*;
+
+#[cfg(windows)]
+mod windows;
+#[cfg(windows)]
+pub use self::windows::*;