Get examples running according to README

[matchai]

Hi there. Love the project!

In trying to run it locally, I saw the examples weren't building and running as described in the README.

Example Logs

plugy/examples/foo-plugin
❯ cargo build --target wasm32-unknown-unknown
   Compiling plugy v0.3.1 (/Users/matchai/dev/plugy)
   Compiling runner v0.3.1 (/Users/matchai/dev/plugy/examples/runner)
error[E0432]: unresolved import `plugy::runtime`
 --> examples/runner/src/lib.rs:2:12
  |
2 | use plugy::runtime::Plugin;
  |            ^^^^^^^ could not find `runtime` in `plugy`

error[E0432]: unresolved import `async_trait`
 --> examples/runner/src/lib.rs:1:5
  |
1 | use async_trait::async_trait;
  |     ^^^^^^^^^^^ use of undeclared crate or module `async_trait`

error[E0432]: unresolved import `xtra`
 --> examples/runner/src/lib.rs:4:5
  |
4 | use xtra::{Address, Handler};
  |     ^^^^ use of undeclared crate or module `xtra`

error[E0433]: failed to resolve: use of undeclared crate or module `xtra`
  --> examples/runner/src/lib.rs:22:19
   |
22 | #[derive(Default, xtra::Actor)]
   |                   ^^^^ use of undeclared crate or module `xtra`

error[E0433]: failed to resolve: use of undeclared crate or module `xtra`
  --> examples/runner/src/lib.rs:33:57
   |
33 | ...x: &mut xtra::Context<Self>) {
   |            ^^^^ use of undeclared crate or module `xtra`

Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `runner` (lib) due to 5 previous errors

The cause was missing dependencies in the wasm-32 build and an import from the runtime feature from within the plugin.

Here's a small PR getting things working as minimally as possible.