Feature request: support negative values 

`-1 * x` will throw the following error 

```
thread 'main' panicked at src/main.rs:61:60:
called `Result::unwrap()` on an `Err` value: UnrecognizedToken { token: (1, Token(7, "-"), 2), expected: ["\"(\"", "r#\"[0-9]+\"#", "r#\"[a-zA-Z]+\"#"] }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```