feat(sort): add props for custom sorting

[jxn-30]

This is a feature proposal that adresses #177

The following new props have been added:

• sort-function
• multi-sort-function

sort-function

takes the following arguments:

• a: Item
• b: Item
• sortBy: string
• sortDesc: boolean
• getItemValue: typeof getItemValueFn
• defaultSortFunction: (a: Item, b: Item) => number

it should return a number, if a nullish (undefined and null) is returned, the default sort function will automatically be applied.

multi-sort-function

takes the following arguments:

• sortByArr: string[]
• sortDescArr: boolean[]
• itemsToSort: Item[]
• index: number

it should return an Item[] (Array of Item-Objects). There is no "fallback to default if undefined returned" as for sort-function to allow more controll over the sorting.