Assign a due date to a task

[JordanAdams]

Is your feature request related to a problem? Please describe.
Right now there's no means of assigning a due date/time to tasks.

Describe the solution you'd like
Allow for setting a due date on tasks and display this alongside them (ideally in a human readable format).

$ tb -t Do something tomorrow due:tomorrow

$ tb -t Do something in 3 days due:3d

$ tb
    My Board [0/4]
      1. ☐  Pay bills (yesterday)  
      2. ☐  Team dinner (tomorrow)
      3. ☐  Approve PR #270 (3 days)

[kalexmills]

If this gets implemented, it should take PR #52 into account also. The --after flag has meaning for dateCreated and also dateDue.

[fioreale]

any updates on this?