`--shuffle` option would be nice to have

Currently, all tests are run in exactly the order of their definition in the source file. Good testing practices require tests to be shuffled, in order to avoid their possible interdependencies. How about adding a new command line option `--shuffle`, with an optional seed number:

```bash
shunit2 --shuffle my-tests.sh
```

With a seed:

```bash
shunit2 --shuffle=12345 my-tests.sh
```