This program can be used to execute imapsync [1] in parallel. This makes
it possible to migrate a lot of mailboxes in a short period of time.

Example usage::

    export SOURCE_MASTER_PASSWORD="supersecretpassword"
    export TARGET_MASTER_PASSWORD="supersecrettoo"

    imapsync-director \
        --source-host \
        --source-username-suffix "*master" \
        --source-password-env SOURCE_MASTER_PASSWORD \
        --target-host \
        --source-username-suffix "*master" \
        --target-password-env TARGET_MASTER_PASSWORD \
        --accounts-file accounts.txt

This will read user names from the file `accounts.txt`
and run 16 (default) imapsync commands in parallel.