Add ability to set a filter prior to initial sync.

Useful for only syncing with a subset of joined rooms or only retrieving certain relevant types of events.

lib/client.js

@@ -2511,6 +2511,9 @@ MatrixClient.prototype.getTurnServers = function() {
  *
  * @param {Number=} opts.pollTimeout The number of milliseconds to wait on /events.
  * Default: 30000 (30 seconds).
+ *
+ * @param {Filter=} opts.filter The filter to apply to /sync calls. This will override
+ * the opts.initialSyncLimit, which would normally result in a timeline limit filter.
  */
 MatrixClient.prototype.startClient = function(opts) {
     if (this.clientRunning) {