Create multiple issues at once with `ejira-heading-to-task`

[nyyManni]

Allow having active region for the call to ejira-heading-to-task and ejira-heading-to-subtask`. Also think if there would be a way to create for example an epic with its issues or a story with its subtasks by having a top-level heading define the summary and description for the epic/story and its children define the issues/subtasks.