Rework merge signature

I found the syntax of calling merge, supplying the first dictionary, and then having a named argument for the remaining dictionaries fairly awkward, any thoughts?

Given dictionaries `dict`, `dict2` and `dict3` previously merge was
called via:
`$.merge(dict, dictionaries: dict2, dict3)`

With these changes merge becomes:
`$.merge(dictionaries: dict, dict2, dict3)`

and gains
`$.merge(arrays: arr, arr2, arr3)`