Attaching to already running process

The idea is to extend curent `Command.Run` API by adding:
```csharp
public static Command AttachTo(int processId)
```
which would internally use [`Process.GetProcessById`](https://docs.microsoft.com/en-gb/dotnet/api/system.diagnostics.process.getprocessbyid?view=netframework-4.7.2#System_Diagnostics_Process_GetProcessById_System_Int32_).

What do you think?