Generate human readable IDs for client URLs (#77)

Based on @FranzSkuffka's suggested enhancement in PR 207 on
localtunnel/localtunnel
(https://github.com/localtunnel/localtunnel/pull/207), this PR uses the
"human-readable-ids" package to generate IDs of the form
"adjective-noun-number". These IDs are easier to remember and arguably
more useful in testing since they can be retyped without copying and
pasting.

Some sample IDs generated by this package are:

* heavy-puma-90
* grump-walrus-64
* purple-starfish-58
* nasty-seahorse-9

lib/ClientManager.js

@@ -4,8 +4,8 @@ import Proxy from './Proxy';
 import on_finished from 'on-finished';
 import http from 'http';
 import pump from 'pump';
+import { hri } from "human-readable-ids";
 
-import rand_id from './rand_id';
 import BindingAgent from './BindingAgent';
 
 const NoOp = () => {};
@@ -36,7 +36,7 @@ class ClientManager {
 
         // can't ask for id already is use
         if (clients[id]) {
-            id = rand_id();
+            id = hri.random();
         }
 
         const popt = {