Support piping code directly into `dotnet run` via stdin

We should consider accepting the C# code from stdin to `dotnet run` as an alternate way to run a file-based app, e.g.:

```shell
> 'Console.WriteLine("Hello, World!");' | dotnet run -
Hello, World!
> 
```

```shell
> curl -S http://superawesome-trustedwebsite.com/install | dotnet run -
Installing super awesome tool.... done!
> 
```