Use Rust for the front-end too (#374)

Rust can compile to WASM, so let's use it for front-end code as well.

To compile the front-end:

```
cargo install cargo-web
cargo web deploy -p plume-front
```

plume-front/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "plume-front"
+version = "0.1.0"
+authors = ["Plume contributors"]
+
+[dependencies]
+stdweb = "0.4"