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.
`field_span` is incorrect, it should be the value's span.
Maybe you could add a function `get_span_from_tts()`, and turn this line into `a => ecx.span_err(get_span_from_tts(a), ...)`