support parsing negative numbers

src/parsing.rs

@@ -24,6 +24,10 @@ pub fn parse_number_with_radix<T: Num>(
 ) -> Option<T> {
     tokens.skip_tokens_while(|t| *t == ' ');
     let mut remaining = limit.map(|x| x as isize).unwrap_or(isize::MAX);
+    let mut prefix = String::new();
+    if tokens.token('-') {
+        prefix.push('-');
+    }
     let digits: String = tokens
         .tokens_while(|c| {
             remaining -= 1;
@@ -31,7 +35,7 @@ pub fn parse_number_with_radix<T: Num>(
         })
         .collect();
     (!digits.is_empty())
-        .then(|| T::from_str_radix(&digits, base).ok())
+        .then(|| T::from_str_radix(&format!("{}{}", prefix, digits), base).ok())
         .flatten()
 }