Support relative dates in add subcommand.

When using the same patterns as for recurrence, you can specify relative
dates instead of absolute ones when adding a task with start or due
dates. These will be automatically converted to absolute dates:

Example:

    todo.sh add "(C) Pick up laundry due:1d"

turns into:

    todo.sh add "(C) Pick up laundry due:2013-08-31"