Add support for HTTP and SOCKS5 proxies (#127)

As mentioned in https://github.com/davegallant/vpngate/issues/126, adding support for proxies will help bypass issues when vpngate.net is not accessible directly.

This works by:

```sh
# http proxy
sudo vpngate connect --proxy "http://localhost:8080"

# socks5 proxy
sudo vpngate connect --socks5 "127.0.0.1:1080"
```

pkg/vpn/list.go

@@ -4,9 +4,12 @@ import (
 	"bytes"
 	"io"
 	"net/http"
+	"net/url"
+	"os"
 
 	"github.com/jszwec/csvutil"
 	"github.com/rs/zerolog/log"
+	"golang.org/x/net/proxy"
 
 	"github.com/davegallant/vpngate/pkg/util"
 	"github.com/juju/errors"
@@ -55,10 +58,11 @@ func parseVpnList(r io.Reader) (*[]Server, error) {
 }
 
 // GetList returns a list of vpn servers
-func GetList() (*[]Server, error) {
+func GetList(httpProxy string, socks5Proxy string) (*[]Server, error) {
 	cacheExpired := vpnListCacheIsExpired()
 
 	var servers *[]Server
+	var client *http.Client
 
 	if !cacheExpired {
 		servers, err := getVpnListCache()
@@ -75,11 +79,43 @@ func GetList() (*[]Server, error) {
 
 	log.Info().Msg("Fetching the latest server list")
 
+	if httpProxy != "" {
+		proxyURL, err := url.Parse(httpProxy)
+		if err != nil {
+			log.Error().Msgf("Error parsing proxy: %s", err)
+			os.Exit(1)
+		}
+		transport := &http.Transport{
+			Proxy: http.ProxyURL(proxyURL),
+		}
+
+		client = &http.Client{
+			Transport: transport,
+		}
+
+	} else if socks5Proxy != "" {
+		dialer, err := proxy.SOCKS5("tcp", socks5Proxy, nil, proxy.Direct)
+		if err != nil {
+			log.Error().Msgf("Error creating SOCKS5 dialer: %v", err)
+			os.Exit(1)
+		}
+
+		httpTransport := &http.Transport{
+			Dial: dialer.Dial,
+		}
+
+		client = &http.Client{
+			Transport: httpTransport,
+		}
+	} else {
+		client = &http.Client{}
+	}
+
 	var r *http.Response
 
 	err := util.Retry(5, 1, func() error {
 		var err error
-		r, err = http.Get(vpnList)
+		r, err = client.Get(vpnList)
 		if err != nil {
 			return err
 		}

pkg/vpn/list_test.go

@@ -9,7 +9,7 @@ import (
 
 // TestGetListReal tests getting the real list of vpn servers
 func TestGetListReal(t *testing.T) {
-	_, err := GetList()
+	_, err := GetList("", "")
 
 	assert.NoError(t, err)
 }