allow dotnet try to interact with long-running programs

[jonsequitur]

The initial release of dotnet try supports only one interaction model with the user's program, which is:

• Start a process (invoking the user's Main and passing arguments from the sample snippet)
• Capture stdout and stderr
• Return buffered stdout and stderr to the user when the process exits.

Support for longer-running programs will add to the kinds of scenarios that dotnet try can support:

• The ability to read input from the user (i.e. Console.ReadLine).
• Display stdout and stderr in realtime while the program is running, e.g. for streaming samples, log output from a web app, etc.
• The ability to start an ASP.NET Core app and launch a second browser to interact with it while it's running.

[HEskandari]

We had a look to see if dotnet try fits our NServiceBus samples/workshops, and one of the main things missing is the ability to run a long-running program (e.g. endpoints) and being able to stop them.