Provide a Cleaner role to enable plugins to run at dzil clean [rt.cpan.org #75426]

[rjbs]

https://rt.cpan.org/Ticket/Display.html?id=75426

This was mentioned briefly on irc
but I thought I'd make a formal request for consideration.

What do you think about a plugin phase for running at the clean command?

A feature request was submitted to the Run plugin dist
for a `Run::Clean` plugin to enable running custom actions when using
the clean command:

https://github.com/Getty/p5-dist-zilla-plugin-run/issues/7

but currently dzil doesn't support any cleaning plugins.