Better repeat time parsing

[2Cubed]

Currently, repeat adding looks like the following:

!repeat add 600 command_name arg1 arg2

However, the time argument, 600, is not very user-friendly here.

• Switch to minutes by default.
  We don't allow for an interval less than 60 seconds anyways, so it's silly to make users multiply the number of minutes by 60 manually.

• Better interval parsing.
  We should parse specially formatted intervals with units. For example, 90s should mean 90 seconds, 20m should mean 20 minutes, 1d should mean one day, etc.

[MrLotU]

I like the idea, kinda related to #268 I think if you were to combine the two that would for some awesome saucy stuff!