Parallelize regiment

[novemberkilo]

In the short term, support usage of regiment sort via xargs. So, Regiment.IO.merge should take a list of temp directories and merge across them.

In the longer term, regiment merge should mimic sort -m

[novemberkilo]

In order to support parallelized regiment, suggest the following approach:

1. Implement regiment shard that takes the same arguments as regiment sort but only performs the operation of constructing a set of sorted temporary files (in the usual binary format) and returns the path to the directory in which these files are created.

2. Implement regiment merge that takes a list of directories (that are the outputs of running regiment shard in parallel) and perform the merge operation on the collection of all temporary sorted files contained within those directories.