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!

Comece localizando o ponto de entrada do tratamento de PR-template neste JavaScript GitHub wrapper e determine se o comportamento de entrada do template é exposto aqui ou pertence ao próprio GitHub. Defina como devem funcionar a substituição de placeholders, os campos obrigatórios, a primeira edição e as edições posteriores; a conclusão exigiria um design acordado e testes para esses comportamentos.

Escrita pelo modelo de indexação a partir do texto da issue.