Removal of temporary `SOURCE` file is no longer performed by default on
completion.  Instead, this feature is performed only if the `CLEANUP`
option is set to 'true'.

The `SOURCE` file and any output files are no longer overwritten on
consecutive script executions. Instead, there are now option flags for
`OVERWRITE_SOURCE` and `OVERWRITE`, where the former overwrites all
files and the later preserves the `SOURCE` file and overwrites only the
output files.

This allows the script to reuse the `SOURCE` file on subsequent
executions by default, so if concatenation was successful, only the
output files (or missing output files) are processed. This will save a
great deal of time if the first execution does not achieve the desired
results.

Minor formmating fixes also added in this commit.

Single-channel export now outputs WAV files, instead of W64.