Added functionality for showing and modifying play mode.

Added command
```
mode
```
```
mode <normal, shuffle_norepeat, shuffle, repeat_all>
```

Perhaps we should print the optional parameters in a list and let it operate like the ```set``` command with ```mode 1``` for ```normal``` , input anyone?

From what I can see on https://github.com/petteraas/socos/blob/46793495abb623b3c1dd79873c73df6544daa197/socos/core.py#L427 help intentionally only prints the first line of __doc__ I'm not sure if this is always a good idea?, but I won't change that in this PR!