feat(tokens): add duration option

[davlgd]

Adds a --duration or -d option to manage API token duration until expiration. Default value is 1y, can be months (m), weeks (w) or days (d). Also ms or s but not documented as it's not very useful. Duration can't be greater than 1 year.

clever create tokens myToken
clever create tokens myToken -d 6m
clever create tokens myToken --duration 2y # Leads to an error

We could not use duration-js here, but it simplifies code.