WasiCtx and preopen

Hi,

`Wasi::new` takes a `WasiCtx` parameter that can be created with environment args as shown in your examples on WASI and linking. With the wasmtime binary you can pass the `--dir` parameter to preopen a directory where the wasm module can read a file from. I did not so far get this to work when instantiating a WASI module using your API. It seems the args passed into the `WasiCtx` creation does not end up with directories being pre-opened. The end result is that file IO from a WASI module does not seem to work unless you are calling it from the wasmtime binary. Maybe I'm missing something? Thanks.