Modernize the doc subcommand

src/cli/doc.rs

@@ -1,4 +1,12 @@
-pub fn doc() -> Result<(), anyhow::Error> {
-    opener::open("https://rojo.space/docs")?;
-    Ok(())
+use structopt::StructOpt;
+
+/// Open Rojo's documentation in your browser.
+#[derive(Debug, StructOpt)]
+pub struct DocCommand {}
+
+impl DocCommand {
+    pub fn run(self) -> anyhow::Result<()> {
+        opener::open("https://rojo.space/docs")?;
+        Ok(())
+    }
 }