Vagrantfile: Add support for a custom post-provision script.

Documentation tweaked by tabbott.

Fixes #3108.

.gitignore

@@ -27,3 +27,4 @@ npm-debug.log
 var/*
 .vscode/
 tools/conf.ini
+tools/custom_provision

Vagrantfile

@@ -101,6 +101,10 @@ if [ -d "/sys/fs/selinux" ]; then
 fi
 ln -nsf /srv/zulip ~/zulip
 /srv/zulip/tools/provision
+if [ -f /srv/zulip/tools/custom_provision ]; then
+  chmod +x /srv/zulip/tools/custom_provision
+  /srv/zulip/tools/custom_provision
+fi
 SCRIPT
 
   config.vm.provision "shell",

docs/dev-env-first-time-contributors.md

@@ -462,6 +462,14 @@ much faster than the original `vagrant up` since the base image is
 already cached on your machine (it takes about 5 minutes to run with a
 fast Internet connection).
 
+Any additional programs (e.g. Zsh, emacs, etc.) or configuration that
+you may have installed in the development environment will be lost
+when you recreate it.  To address this, you can create a script called
+`tools/custom_provision` in your Zulip Git checkout; and place any
+extra setup commands there.  Vagrant will run `tools/custom_provision`
+every time you run `vagrant provision` (or create a Vagrant guest via
+`vagrant up`).
+
 #### Shutting down the development environment for use later
 
 To shut down but preserve the development environment so you can use