config: Add calloop feature to provide a calloop source (#102)

This should be useful for integrating cosmic-config into things using
calloop, like cosmic-comp.

cosmic-config/src/lib.rs

@@ -7,6 +7,9 @@ use std::{
     sync::Mutex,
 };
 
+#[cfg(feature = "calloop")]
+pub mod calloop;
+
 #[derive(Debug)]
 pub enum Error {
     AtomicWrites(atomicwrites::Error<std::io::Error>),