List choices for --mode and make --pprof a path, not just a bool.

rtiow/src/renderer.rs

@@ -78,10 +78,11 @@ pub struct Opt {
     /// Sub-samples per pixel
     #[structopt(short = "s", long = "subsample", default_value = "1")]
     pub subsamples: usize,
+    /// Select scene to render, one of: "bench", "book", "tutorial" "cube", "bvh", "test"
     #[structopt(long = "model", default_value = "test")]
     pub model: Model,
-    #[structopt(long = "pprof")]
-    pub pprof: bool,
+    #[structopt(long = "pprof", parse(from_os_str))]
+    pub pprof: Option<PathBuf>,
     /// Use acceleration data structure, may be BVH or kd-tree depending on scene.
     #[structopt(long = "use_accel")]
     pub use_accel: bool,