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.
The delimiters are part of the vector, so you could recursively call `TTDelim(delim) => get_span_from_tts(delim[0])`