Add support for grouping types

Suggestion for a new option `--group` which groups types (functions, events, constants etc).

Today, given JSdoc
```
/**
* @constant SESSION
*/
/**
* @event my:event
*/
/**
* Update account data to backend. Also updates local user data if successful.
* @methodOf platform.model.user.User
* @param {Object} payload Payload according to spec
* @return {Promise} Promise of new user data.
*/
function updateAccountData(payload) {}
```

The output in markdown becomes

---

### SESSION
### my:event
### updateAccountData

---

Making no visual indication that these are of different types. I suggest to add config option `--group` which could generate the following:

----

### Constants
#### SESSION
### Events
#### my:event
### Methods
#### updateAccountData

---