* Don't use joiner string when prefix is empty.
* Add option to specify joiner string.
* Add option to not URL-escape file names.

I **_thought_** it'd be cool to apply URL-escaping to the file name prefix as well, just in case it included spaces or other characters.  I forgot that a common use for the prefix is to specify a directory path that will contain the file.  When the slashes ("`/`") of the path are URL-escaped, they become "`%2F`" and the directories will not be created.  So, I moved the prefix handling code to come after the URL-escaping.

Trying to make the `return` statement cleaner, I thought it'd be good to add the file name suffix immediately after the file name is URL-escaped.  I'd forgotten that the suffix will not be added if the new `-e` option is used to skip URL-escaping.  So, I put the suffix back where I had it.

Not strictly part of this issue, but as I was checking for docs that I should update as a result of my changes, I noticed this document showed how to split data using the `put` and `tee` combination, but not about the `split` verb.

When I ran `make dev`, generating `data-diving-examples.md` failed.  The two `manpage.txt` files ended up empty, but `mlr.1` seems to be correct.