Option to group output from `stack ls dependencies --license` for readability

Would like to group the dependencies by license from `stack ls dependencies --license`. This allows for greater readability.

It is currently in the form of:
```
$ stack ls dependencies --license
happy BSD2
ansi-terminal BSD3
array BSD3
hashable BSD3
wai MIT
wai-extra MIT
...
```

Suggest adding flag to sort this output into groups

```
$ stack ls dependencies --license --group
BSD2: happy
BSD3: array, hashable, ansi-terminal
MIT: wait, wai-extra
...
```
Am happy to open a PR if others find this helpful as well. Thanks!