Add getRecommendedShards and automatic shard count in ShardingManager (#796)

* draft stuff

fix docstring for Client#token

Reorganise resolver

make env better for shards, clean up docs

Fix Gus' log messages

7

meh just gateway/bot not v7 :(

final changes, ready for mergin!

build docs

make default totalShards 'auto', fix docs for totalShards type

clean up docs more

run docs

* make consistancy real

* Update and rename getRecommendedShards.js to GetRecommendedShards.js

* Update GetRecommendedShards.js

* Update index.js

* Update RESTMethods.js

* Update Shard.js

* Update GetRecommendedShards.js

* Update ShardingManager.js

* run docs

src/util/GetRecommendedShards.js

@@ -0,0 +1,19 @@
+const superagent = require('superagent');
+const botGateway = require('./Constants').Endpoints.botGateway;
+
+/**
+ * Gets the recommended shard count from Discord
+ * @param {number} token Discord auth token
+ * @returns {Promise<number>} the recommended number of shards
+ */
+module.exports = function getRecommendedShards(token) {
+  return new Promise((resolve, reject) => {
+    if (!token) reject(new Error('A token must be provided.'));
+    superagent.get(botGateway)
+      .set('Authorization', `Bot ${token.replace('Bot ', '')}`)
+      .end((err, res) => {
+        if (err) reject(err);
+        resolve(res.body.shards);
+      });
+  });
+};