Add "last_month" shortcut

[demisx]

Similar to "this_month", please add "last_month" shortcut. It's often used to invoice for last month work. My understanding it would go into https://github.com/kriskbx/gitlab-time-tracker/blob/master/src/gtt-report.js#L128-L140 data shortcuts section:

if (program.last_month)
    config
        .set('from', moment().subtract(1, 'months').startOf('month'))
        .set('to', moment().subtract(1, 'months').endOf('month'));