Add the folder structure for the toml config.

src/config/definition.rs

@@ -0,0 +1,4 @@
+/// This is the struct for the writer.toml config file that ends up in each site's root directory.
+pub(crate) struct Config {
+    foo: String,
+}

src/config/mod.rs

@@ -0,0 +1,2 @@
+mod definition;
+pub(crate) use definition::Config;