make sharing to clipboard a feature that can be disabled

this is for users that want to eliminate dependences (e.g. libxcb).
the feature is enabled by default and must be explicitly disabled.

fixes #84

src/main.rs

@@ -2,6 +2,7 @@ extern crate clap;
 extern crate crossbeam_channel;
 #[macro_use]
 extern crate cursive;
+#[cfg(feature = "share_clipboard")]
 extern crate clipboard;
 extern crate directories;
 extern crate failure;