[feature] Support add(n, 'isoweek') and subtract(n, 'isoweek')

[thomashusa]

Description of the Issue and Steps to Reproduce:
Currently adding or subtracting 'isoweek' from a moment returns the same moment:

> m = moment()
moment("2018-03-28T13:12:35.901")
> m.add(1, 'isoweek')
moment("2018-03-28T13:12:35.901")

It would be convenient if isoweek was aliased to week in this use case so that the following would be returned:

> m = moment()
moment("2018-03-28T13:12:35.901")
> m.add(1, 'isoweek')
moment("2018-04-04T13:12:35.901")

The reason behind the request is that frequently have to "chunk" time series data by a user provided period (such as a week). moment.js makes it very simple to find the start of a chunk by doing something like moment(timestamp).startOf(period). However, if the period is isoweek, I can not simply calculate the next chunk as usual via moment.add(1, period) since this will return the same timestamp and not a timestamp 1 week in the future.

Environment:
Node.JS 8.9.4 on Ubuntu 16.0.4

[icambron]

I think this is a reasonable request, since we'd just make it do the same thing as week in add and subtract.

[adamreisnz]

I want to +1 this, but at the same time I'm really annoyed that moment chose to start weeks on a sunday by default and not simply use the iso default. That would make everyone's lives a lot easier:

const period = 'week';
moment().startOf(period).subtract(1, period);

Oops doesn't work as intended, I get sunday instead of monday.

const period = 'isoWeek';
moment().startOf(period).subtract(1, period);

Oops doesn't work as intended, moment doesn't know how to handle subtraction of an iso week 🤦‍♂️