re-add the "socks" feature (using tokio-socks) (#769)

The "socks" feature has been removed for a while now, the optional
dependency on the "socks" crate commented out.

The code for actually providing the socks feature was, however, still
mostly present, if a bit out of date.

This commit re-adds the socks feature using the tokio-socks (instead of
socks) crate.

Closes #620

examples/tor_socks.rs

@@ -0,0 +1,24 @@
+#![deny(warnings)]
+
+// This is using the `tokio` runtime. You'll need the following dependency:
+//
+// `tokio = { version = "0.2", features = ["macros"] }`
+#[tokio::main]
+async fn main() -> Result<(), reqwest::Error> {
+    // Make sure you are running tor and this is your socks port
+    let proxy = reqwest::Proxy::all("socks5://127.0.0.1:9050").expect("tor proxy should be there");
+    let client = reqwest::Client::builder()
+        .proxy(proxy)
+        .build()
+        .expect("should be able to build reqwest client");
+
+    let res = client.get("https://check.torproject.org").send().await?;
+    println!("Status: {}", res.status());
+
+    let text = res.text().await?;
+    let is_tor = text.contains("Congratulations. This browser is configured to use Tor.");
+    println!("Is Tor: {}", is_tor);
+    assert!(is_tor);
+
+    Ok(())
+}