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/Cargo.toml

@@ -7,6 +7,7 @@ description = "Raw FFI bindings to implot"
 license = "MIT/Apache-2.0"
 categories = ["gui", "external-ffi-bindings"]
 build = "build.rs"
+links = "implot"
 
 [dependencies]
 imgui-sys = { git = "https://github.com/Gekkio/imgui-rs/", branch = "master" }