feat: options.PouchDB

BREAKING CHANGE:

Before, `options.db` had to be passed to the Api constructor, pre-initialised with `"pouchdb-users"`:

```js
var AccountApi = require("@hoodie/account-server-api")
var PouchDB = require("pouchdb")

PouchDB.plugin(require("pouchdb-users"))

var db = new PouchDB("http://localhost:5984/_users")

db.installUsersBehavior().then(function () {
  var api = new AccountApi({
    db: db,
    secret: "secret123"
  })

  api.accounts.findAll().then(logAccountStats)
  api.accounts.on("change", logAccountChange)
})
```

Now, `options.PouchDB` is enough. Optionally `options.usersDb` for a custom users db name can be passed (it defaults to `_users`):

```js
var AccountApi = require("@hoodie/account-server-api")
var PouchDB = require("pouchdb")
  .plugin(require("pouchdb-users"))

var api = new AccountApi({
  PouchDB: PouchDB,
  usersDb: "my-users-db",
  secret: "secret123"
})
```

index.js

@@ -18,9 +18,10 @@ var accountsOn = require('./lib/accounts/on')
 var startListeningToAccountChanges = require('./lib/utils/start-listening-to-account-changes')
 
 function accountApi (options) {
+  options.PouchDB.plugin(require('pouchdb-users'))
   var accountsEmitter = new EventEmitter()
   var state = {
-    db: options.db,
+    db: new options.PouchDB(options.usersDb || '_users'),
     secret: options.secret,
     accountsEmitter: accountsEmitter
   }

lib/setup.js

@@ -10,4 +10,8 @@ function setup (state) {
       throw error
     }
   })
+
+  .then(function () {
+    return state.db.installUsersBehavior()
+  })
 }