Fix negative number syntax. Add a unary '-' operator.

Closes #63.

lexer.l

@@ -73,7 +73,7 @@ struct lexer_param;
   yylval->literal = jv_string_sized(yytext + 1, yyleng - 1); return FORMAT;
 }
 
--?[0-9.]+([eE][+-]?[0-9]+)? { 
+[0-9.]+([eE][+-]?[0-9]+)? { 
    yylval->literal = jv_parse_sized(yytext, yyleng); return LITERAL; 
 }