Set up plugin structure

src/main.rs

@@ -1,3 +1,36 @@
-fn main() {
-    println!("Hello, world!");
+use age_plugin::run_state_machine;
+use gumdrop::Options;
+
+mod error;
+mod plugin;
+
+use error::Error;
+
+#[derive(Debug, Options)]
+struct PluginOptions {
+    #[options(help = "Print this help message and exit.")]
+    help: bool,
+
+    #[options(
+        help = "Run the given age plugin state machine. Internal use only.",
+        meta = "STATE-MACHINE",
+        no_short
+    )]
+    age_plugin: Option<String>,
+}
+
+fn main() -> Result<(), Error> {
+    let opts = PluginOptions::parse_args_default_or_exit();
+
+    if let Some(state_machine) = opts.age_plugin {
+        run_state_machine(
+            &state_machine,
+            || plugin::RecipientPlugin::default(),
+            || plugin::IdentityPlugin::default(),
+        )?;
+        Ok(())
+    } else {
+        // TODO: CLI identity generation
+        Ok(())
+    }
 }