carboncopy: Drop emails from threads being watched into special CC folder.

Many times you are posting some email to a mailing list. Usually you want
to track replies to your email, as well if someone replies to another email
in the thread, posted by another person.

If you are using procmail, you may have it configured to store mails into
separate folders for mailing lists, just to keep yourself organized. However,
you are forced to check those folders periodically, to see if there are new 
replies to your emails.

Procmail itself doesn't provide any functionality to track email threads and
notify you if a new email is posted to a thread you've started. So this simple
tool allows to check if email was posted in reply to certain one in the thread.

With simple procmail configuration, like below

:0 cW
* ? /path/to/executable
/home/user/Maildir/CC/

emails will be copied into CC folder, if they are replies to a thread you'd 
started.

You have to configure $HOME/.ccrc file as following

cc_header_file = /home/user/.ccheaders
originator_email = user@domain.com

where 

cc_header_file - full path to the database of message IDs
originator_email - email of sender you want to "Watch" for replies automatically.
                   Usually this is your own email address.

How this works: every email should have 2 headers:

- Message-Id - the unique ID of email message, generated by MTA
- In-Reply-To - ID of email, which was replied by current one

So the task of the current tool is to check, if there is a value of In-Reply-To header 
present in the email database (plain text file), and if so - add the current message ID
to the database and exit with code 0.

We have to add current message ID to make it possible follow-ups of the current email
will be tracked as well.

If email message contains originator_email from settings file, then current ID of the
email will be added to the storage as well. So follow-ups will be tracked as well