Support ability to disable ampersat (@) parsing

Support a flag to disable ampersat/file parse support; required when options actually begin with an ampersat symbol and you don't intend for it to be a file.

e.g.

```java
            String[] args = new String[]{"--name", "@tzellman"};
            Params params = new Params();
            JCommander jc = new JCommander(params);
            jc.setExpandAmpersat(false);
            jc.parse(args);
```