fix build warning by making `options` immutable #47

fixes this warning:

warning: variable does not need to be mutable
   --> src/main.rs:140:9
    |
140 |     let mut options = vec![
    |         ----^^^^^^^
    |         |
    |         help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` (part of `#[warn(unused)]`) on by default