Allow setting fixed first properties to be sorted first

[Thodor12]

Add a new option called xmlSortAttributesFirst, this allows to sort attributes in a fixed way. For example if I were to have an id field on my XML and I always want this attribute to be sorted first, I can put xmlSortAttributesFirst: ["id"] to force this property to come first.

The attributes in this list are sorted in the order they are listed, even if certain are not present.

I've added 2 tests to ensure the functionality works as intended.