Add xf_format writing support

[LLBlumire]

This is a significantly breaking change, and would require a version bump.

The basic idea is to have an XfFormat attached to a cells data, instead of a NumberFormat. This change is breaking, as where before people could create a number format and place that on their cell directly, they now have to make a number format, attach that number format to an xf format, and then put that xf format on their cell.

This currently only adds borders and fills, but it adds clear room in the API to add other xf formats in future.