Got initial unsafe bindings working. See comments.

- This now builds the bindings (still with cbindgen in build.rs,
  will change that in a bit) and links to them successfully. Everything
  currently points to the master branch at Gekkio/imgui-rs, because
  there has not been a release yet with the PR merged that exposes
  the defines in the sys crate.

- I have now included a little demo crate that calls into the unsafe
  generated bindings as a proof of concept. Next there will need to
  be safe and more Rust-y bindings written around this, just like
  imgui-rs has them.

implot-sys/src/lib.rs

@@ -1,3 +1,7 @@
-fn _does_nothing() {
-    println!("This does nothing yet");
-}
+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+
+// TODO(4bb4) change this to include the bindings we hand-generate
+// once that is happening
+include!(concat!(env!("OUT_DIR"), "/bindings.rs"));