Improve generate_gl_bindings! plugin syntax

The new syntax looks like:

~~~rust
generate_gl_bindings! {
    api: "gl",
    profile: "core",
    version: "4.5",
    generator: "global",
    extensions: [
        "GL_EXT_texture_filter_anisotropic",
    ],
}
~~~

The updated README and doc comment should supply you with more information.