Added a buffered mode

[asaushkin]

Added ability to send buffered messages. I use this feature for sending reports from PostgreSQL. While sending with this option we will not to create temporary files and will not to send a every line in a separate message.

Like this:

psql -X -h redmine redmine -f /home/ags/workspace/crontab/check_redmine_open_issue.sql | slackcat -b