Add support for websockets (#3)

Basically a copy/paste of whats in warp.

Example usage:

```rust
use tower_web::{prelude::*, ws::{ws, WebSocket}};

let app = route("/ws", ws(handle_socket));

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = msg.unwrap();
        socket.send(msg).await.unwrap();
    }
}
```

Cargo.toml

@@ -12,6 +12,9 @@ readme = "README.md"
 repository = "https://github.com/davidpdrsn/tower-web"
 version = "0.1.0"
 
+[features]
+ws = ["tokio-tungstenite", "sha-1", "base64"]
+
 [dependencies]
 async-trait = "0.1"
 bytes = "1.0"
@@ -28,16 +31,32 @@ tokio = { version = "1", features = ["time"] }
 tower = { version = "0.4", features = ["util", "buffer"] }
 tower-http = { version = "0.1", features = ["add-extension"] }
 
+# optional dependencies
+tokio-tungstenite = { optional = true, version = "0.14" }
+sha-1 = { optional = true, version = "0.9.6" }
+base64 = { optional = true, version = "0.13" }
+
 [dev-dependencies]
 hyper = { version = "0.14", features = ["full"] }
 reqwest = { version = "0.11", features = ["json", "stream"] }
 serde = { version = "1.0", features = ["derive"] }
 tokio = { version = "1.6.1", features = ["macros", "rt", "rt-multi-thread"] }
-tower = { version = "0.4", features = ["util", "make", "timeout", "limit", "load-shed", "steer"] }
 tracing = "0.1"
 tracing-subscriber = "0.2"
 uuid = "0.8"
 
+[dev-dependencies.tower]
+version = "0.4"
+features = [
+    "util",
+    "make",
+    "timeout",
+    "limit",
+    "load-shed",
+    "steer",
+    "filter",
+]
+
 [dev-dependencies.tower-http]
 version = "0.1"
 features = [