Regression xirr does not ensure cash flows are sorted

[okomarov]

In 2.0.3 we've got:

xirr([
  {amount: -50000, date: new Date('2019/01/01')},
  {amount: -50000, date: new Date('2020/01/01')},
  {amount: 162500, date: new Date('2021/01/01')}
])

> { days: 732, rate: 0.0008630264819782241 }

xirr([
  {amount: -50000, date: new Date('2020/01/01')},
  {amount: -50000, date: new Date('2019/01/01')},
  {amount: 162500, date: new Date('2021/01/01')}
])

> { days: 732, rate: 0.0008630264819782241 }

While in 2.0.5, the second case, i.e. 2019 after 2020, the result changes to:

{ days: 732, rate:   0.003225559800975386 }

[alias-rahil]

@eric-malachias @okomarov please take a look at #20