Feature request: support negative values

[0xicingdeath]

-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