Wasmtime Settings Machine Readable Output

#### Feature

currently running `wasmtime settings` displays a human readable output. I propose adding a switch `--machine_readable` which changes the output to something more convenient to parse (e.g. JSON).

#### Benefit

If wasmtime is being used by other processes (e.g. our usecase has the Unity editor invoking wasmtime to precompile wasm) a machine readable output would be much more convenient to work with.

#### Implementation

Add this:

```
/// Switch output format to machine readable
#[clap(long)]
machine_readable: bool,
```

into the `SettingsCommand` struct. Modify `SettingsCommand.execute` to use a different display system if true.

I'm willing to open a PR for this, if it's a feature that would be accepted.