feat(additional-units): add additional units

[richrdkng]

Add more units with configurable durations (e.g.: how long a year should actually be - 365 days exactly, or 365.25 days, or just use from option to calculate from):

• months ('month', 'months')
• quarters ('q', 'qrt', 'quarter', 'quarters')
• years ('y', 'yr', 'year', 'years')

For example, if 1 year is given, by default, calculate a year from the time of calling the function, e.g.: duration('1 year') is called on 2020-01-30 12:00:00, in this case 1 year won't be ~365.25 days, but calculate the actual milliseconds between 2020-01-30 12:00:00 and 2021-01-30 12:00:00 - (in this case it's 366 days === 31622400000 milliseconds).

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Also consider providing a calculate duration from option, e.g.:

// default value
duration('1 year', { from: 'now' })

// custom date (string as human-readable date)
duration('1 year', { from: '2020-01-30 12:00:00' })

// custom date (a number in milliseconds? or seconds? from epoch) - consider ? later
duration('1 year', { from: 1580385600000 }) // 2020-01-30 12:00:00 GMT in epoch milliseconds