Add an aggregate command

[marcaddeo]

We should have an aggregate command that takes the items from the .cl directory, and aggregates them all into the CHANGELOG.md's Unreleased section.

Currently using this in our CI:

  $CLPARSE CHANGELOG.md -f json | cat - <($CL -f json) | \
    $JQ -s ".[0].releases[0].changes+=.[1] | .[0]" | \
    $CLPARSE - > CHANGELOG.md.new \
    && mv CHANGELOG.md.new CHANGELOG.md

  find .cl -name "*.yml" -type f -delete

But it no longer works because cl now displays changes from the Unreleased section, as well as the .cl directory. So whenever you run the command above, it causes the CHANGELOG to duplicate all entries.