Alternate filename formatting for split

[sloanlance]

I recently enjoyed using split to break a large TSV file into several smaller ones using the -g option to group by field values. Using basic shell tools took a few steps to do this, but mlr could do it all in one. However, when mlr was done, I still had to write a loop to rename all the output files. It is…

# Fix file name formatting.
# Remove mlr's "split_" prefix, change "_" and "+" back to spaces.

for filename in split_*.tsv; do
  newFilename="$(sed -e 's/^split_//' -e 's/[+_]/ /g' <<< "${filename}")"
  mv "${filename}" "${newFilename}"
done

What I'd like to see split do better is…

1. When --prefix '' is specified, DO NOT start the output filenames with underscore. I.e., do not name the files _col+a+value_col+b+value.tsv.
2. DO NOT use underscores between parts of the filename.
3. DO NOT replace spaces with + signs.

To address those, I would change how the prefix is handled (item 1) and add an option to not change spaces to other characters (items 2 and 3).

I'm not very experienced with go, but I'm keen to learn, so I would be happy to work on a solution for this and open a PR. If this is deemed to be a legitimate issue, that is.

[johnkerl]

@sloanlance I'd love to look at a PR! :)