Add MAILTO option to convert_crontab task

Less manual editing for reinstallers.

Existing backwards-compatibility:

    $ bundle exec rake config_files:convert_crontab \
        DEPLOY_USER=deploy \
        VHOST_DIR=/home/vagrant/ \
        VCSPATH=alaveteli \
        SITE=alaveteli \
        CRONTAB=config/crontab-example | grep 'MAILTO='
    MAILTO=cron-alaveteli@mysociety.org

New optional MAILTO option:

     $ bundle exec rake config_files:convert_crontab \
         DEPLOY_USER=deploy \
         VHOST_DIR=/home/vagrant/ \
         VCSPATH=alaveteli \
         SITE=alaveteli
         CRONTAB=config/crontab-example \
         MAILTO=cron@example.org | grep 'MAILTO='
     MAILTO=cron@example.org