Handle more duration options

[defelmnq]

Serpent's current way to handle time.Duration is directly mapped to how Golang itself is handling it.

As Serpent is a CLI, used mostly with flags, it can lead to situations like --flag 720h for a month or --flag 8760h for a year.

We could add some custom options to make it more readable :

1d for a day
1y for a year

(open to more options)

• To keep it simple, we can use the time package and time.AddDate() function to ensure consistency.