Add si_identifier and si_identifier_type to Contact resource
Add si_identifier and si_identifier_type to Contact resource

The Moneybird gem kept warning me about:
```
W, [2019-11-04T13:02:33.800623 #24299]  WARN -- : Moneybird::Resource::Contact does not have an `si_identifier' attribute
W, [2019-11-04T13:02:33.800687 #24299]  WARN -- : Moneybird::Resource::Contact does not have an `si_identifier_type' attribute
```

The fields are used for Simplerinvoicing. https://www.moneybird.nl/simplerinvoicing/
I dont need them, but the warning was pretty annoying :)
maartenvanvliet/moneybird,maartenvanvliet/moneybird