Allow sending of output to a named file

[claremacrae]

Currently, the output is only ever written to standard output.

I often find I want to convert [somefilename]/.md to [somefilename]/.wiki - and it's tedious having to specify the base filename twice on the command line, to redirect output to a file.

It would be useful to supply some kind of command-line argument that makes the script generate a sensible output filename, and write to it.

[claremacrae]

Closing as "won't fix" as I'm not using dokuwiki any more, so no plans to do more work on this repo.
Contributions welcome though.