feat: Support optional env secrets

[Zeko369]

Closes #359

This PR allows specifying optional env secrets with - FOO? syntax

Example

In the following example the app won't build without RAILS_MASTER_KEY env variable being provided but will work both with and without FOO?

env:
  secrets:
    - RAILS_MASTER_KEY
    - FOO?

Todos

• Think about overriding (i.e. if we have FOO in deploy.yml and FOO? in deploy.stage.yml, what's the output) comment here
• Finish specs

[xiaohui-zhangxh]

Good idea, now I have to write them like this:

env:
  secrets:
    - RAILS_MASTER_KEY
  <% if ENV['FOO'].to_s != '' %>
    - FOO
  <% end %>