feat(cli): basic usage of docopts

For now we just show it works within our generator.
Next step is to actually generate docopts grammar.

src/mako/cli/main.rs.mako

@@ -1,3 +1,30 @@
+#![feature(plugin)]
+#![plugin(docopt_macros)]
+
+extern crate docopt;
+extern crate rustc_serialize;
+
+docopt!(Args derive Debug, "
+Usage: rtrace [options] (<OUTPUT-FILE>|-)
+       rtrace --help
+
+Options:
+--width <X>                    The width of the output image [default: 1024]
+--height <Y>                   The height of the output image [default: 1024]
+--samples-per-pixel <SAMPLES>  Amount of samples per pixel. 4 means 16 over-samples [default: 1]
+--num-cores <NUM_CORES>        Amount of cores to do the rendering on [default: 1]
+                               If this is not set, you may also use the RTRACEMAXPROCS
+                               environment variable, e.g. RTRACEMAXPROCS=4.
+                               The commandline always overrides environment variables.
+
+<OUTPUT-FILE>|-     Either a file with .tga extension, or - to write file to stdout
+"
+, flag_samples_per_pixel: u16
+, flag_height: u16
+, flag_width: u16
+, flag_num_cores: usize);
+
 fn main() {
+    let _: Args = Args::docopt().decode().unwrap_or_else(|e| e.exit());
     println!("Hello, ${id} !");
 }