Add support for grouping types

[simonauner]

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

---

[tmcw]

Would this be better as default behavior instead of an option? If the type of documented items is confusing, we should fix that problem always rather than as an option.