PoC: Write current path to tmp file for editing

This is was written as a throw-away-feature, very useful and timesaving for
sitewide copy-edits.

Then if you combine it with the script in the link, you could open the content
page you are looking at in the browser just by a click of a button:

https://raw.githubusercontent.com/bjornerik/bepsays.com/master/script/hugo_edit.sh

Of course this implementation has some limitations, but nothing that should be
very hard to improve upon:

* The naming convention used works for me, but will break soon enough. I guess
some mapping is in order here ...
* Should only be run in dev mode ...