Add Templating Support

[oluceps]

Summary

The templating function acts the same as sops-nix.

templates = {
  test = {
    name = "template.txt";
    content = "this is a template for testing ${config.vaultix.placeholder.example}";
  };
}

2b noticed that the program remains the trailing \n of the secret file(if it exist), which may cause the output not as expected.

Support for inserting multiple secrets into a single template.