The so-called @-syntax for file-based-option-passing

### At-syntax for fetching option values from files

This pull request brings the At-syntax from JCommander to args4j. The syntax is depicted here:

```
java Cmd @/tmp/opts
```

@/tmp/opts is a text file with the options separated by line breaks:

```
-key
value
-variable
value
```
The JCommander version can be viewed here:

http://jcommander.org/#Syntax

### Where does this help

This helps in

* Passing passwords without seeing them in the commandline directly
* Passing complex options that are long and/or contain special characters