Implement automatic cadence

[tmthyjames]

• option for cards to reset daily, every other day, MWF, TTh, weekend, etc (cadence)
  ° 1 - every day
  ° 2 - every other day
  ° 'mwf' - mon wed fri
  ° n - every n days
  ° 0.5 - morning and evening

Is it best to specify:
• 'every_other_day'
• 'mwf'
• 'tr'
• etc

or to use integers:
• 1 - every day
• 2 - every two days, etc
• 1/2 - twice a day
• 1/3 - three times a day, etc

or a mix of both specifications.