Implement External MergeSort

[Mattamorphic]

As a user I want to be able to sort my data sources content and output this as a new file, in potentially a different format.

We can use the merge sort algorithm to break the problem down.
An external merge sort uses smaller files instead of dumping the whole file into memory

[Mattamorphic]

I've implemented a solution to this that only uses 65~ mb of memory at peak usage - however it does take 90 seconds to process a 30~ mb file 👎

I'll need to look into this further - optimisation wise for 0.0.7