Add post-install instructions when using Samples

[mg-stripe]

Problem

Creating a local copy of a Stripe Sample suggests:

$ stripe samples create accept-a-card-payment
...
✔ Selected server: node
✔ Files copied
✔ Project configured
You're all set. To get started: cd accept-a-card-payment

However, we don't describe how to start the server. We should add a post-install message for Samples when created via the CLI.

Feature

Show a more complete set of instructions and allow custom messages to be set in the Sample configuration.

Examples

$ stripe samples create accept-a-card-payment
...
✔ Selected server: node
✔ Files copied
✔ Project configured
You're all set. To get started, run:
  cd accept-a-card-payment
  npm install && npm start

[polybuildr]

Hello! :wave: I noticed this was marked as a good first issue so I thought I'd take a look at what it involves. Please let me know if this isn't meant for an external contributor to help with, though!

I seem to have not understood exactly what the scope of this issue is, so I'll list down what I've learnt.

• Feature request: The samples create subcommand should also show some instructions on how to start the server.
• Samples have a .cli.json file that already supports post-install messages since #243.
• The postInstall["message"] field is already printed out at the end of create.go.

I can see that the server startup instructions would be different based on what integrations the user picks, so is this about how to change the post-install message support to account for this?