Add push command to insert a change to the Unreleased section

[charphi]

Inserting a change can be tedious, so let's automate it!

Draft command:
heylogs push added -m "Add check on GitHub Pull Request links [#173](https://github.com/nbbrd/heylogs/issues/173)"

Or even better with the Git host API:
heylogs push added --issue 173

Before:

# Changelog
...

## [Unreleased]

### Added

- Add check on GitHub mentions of people and teams [#157](https://github.com/nbbrd/heylogs/issues/157)
...

After:

# Changelog
...

## [Unreleased]

### Added

- Add check on GitHub mentions of people and teams [#157](https://github.com/nbbrd/heylogs/issues/157)
- Add check on GitHub Pull Request links [#173](https://github.com/nbbrd/heylogs/issues/173)
...