feat(custom_layout): add yaml file support

This commit adds support for loading custom layouts from yaml files, and
also moves the custom layout loading and validating logic into the
komorebi-core crate.

re #50

komorebic/src/main.rs

@@ -299,7 +299,7 @@ struct Load {
 
 #[derive(Clap, AhkFunction)]
 struct LoadLayout {
-    /// File from which the custom layout definition should be loaded
+    /// JSON or YAML file from which the custom layout definition should be loaded
     path: String,
 }