PR templates - support for inputs

Hi,

This is a request for a feature:
I'd like to have a possibility to specify a fields, that needs to be/or can be entered easily in a PR template.

For example, I want a developer to link a Jira issue in a PR it is related to (we have it in commit, ideally in each one, but it easier to have it in PR description).

So similarly to simple checkbox, I'd like to have easy way in "Preview" tab for PR authors to enter required inputs.

```markdown
Jira issue: {TODO}
```

Maybe a first version might be as simple as this: the text between `{}` will be replaced (so it will work first time only) on first edit, Maybe someone could extend this for edits.

Also I'd like to have an option to mark it mandatory, for example

```markdown
Jira issue: {*TODO}
```

might be a version for mandatory input, so only draft PR can be created if not filled in.

Thanks!

まず、この JavaScript GitHub wrapper における PR-template 処理のエントリーポイントを見つけ、テンプレート入力の動作がここで公開されているのか、それとも GitHub 自体に属するのかを確認します。プレースホルダーの置換、必須フィールド、初回編集、後続の編集がどのように機能すべきかを定義します。完了には、これらの動作について合意された設計とテストが必要です。

索引モデルが issue の本文から書いたものです。