Add support for using and configuring vagrant-proxyconf.

Users can now specify proxy settings for vagrant in a new
~/.zulip-vagrant-config configuration file.

README.dev.md

@@ -90,6 +90,29 @@ environment][using-dev].
 
 [using-dev]: #using-the-development-environment
 
+## Specifying a proxy
+
+If you need to use a proxy server to access the Internet, you will
+need to specify the proxy settings before running `Vagrant up`.
+First, install the Vagrant plugin `vagrant-proxyconf`:
+
+```
+vagrant plugin install vagrant-proxyconf.
+```
+
+Then create `~/.zulip-vagrant-config` and add the following lines to
+it (with the appropriate values in it for your proxy):
+
+```
+HTTP_PROXY http://proxy_host:port
+HTTPS_PROXY http://proxy_host:port
+NO_PROXY localhost,127.0.0.1,.example.com
+
+```
+
+Now run `vagrant up` in your terminal to install the development
+server. If you ran `vagrant up` before and failed, you'll need to run
+`vagrant destroy` first to clean up the failed installation.
 
 Using provision.py without Vagrant
 ----------------------------------

Vagrantfile

@@ -18,6 +18,34 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   config.vm.synced_folder ".", "/vagrant", disabled: true
   config.vm.synced_folder ".", "/srv/zulip"
 
+  proxy_config_file = ENV['HOME'] + "/.zulip-vagrant-config"
+  if File.file?(proxy_config_file)
+    http_proxy = https_proxy = no_proxy = ""
+
+    IO.foreach(proxy_config_file) do |line|
+      line.chomp!
+      key, value = line.split(nil, 2)
+      case key
+      when /^([#;]|$)/; # ignore comments
+      when "HTTP_PROXY"; http_proxy = value
+      when "HTTPS_PROXY"; https_proxy = value
+      when "NO_PROXY"; no_proxy = value
+      end
+    end
+
+    if Vagrant.has_plugin?("vagrant-proxyconf")
+      if http_proxy != ""
+        config.proxy.http = http_proxy
+      end
+      if https_proxy != ""
+        config.proxy.https = https_proxy
+      end
+      if https_proxy != ""
+        config.proxy.no_proxy = no_proxy
+      end
+    end
+  end
+
   # Specify LXC provider before VirtualBox provider so it's preferred.
   config.vm.provider "lxc" do |lxc|
     if command? "lxc-ls"