Add support for custom pre-run and post-run scripts

[Chronial]

We have two PRs (#26, #46) that want to run specific non-snapraid functionality before / after the snapraid run itself. I consider these specific things clearly out of scope for snapraid-runner, but having a way to run a custom command within snapraid-runner's logging/email umbrella seems a rather sensible thing to have.

This could probably be solved by having two "pre-run-command", "post-run-command" config options.

[kyleinprogress]

Just wanted to add my +1 as support for this functionality.

I've started having issues where the process fails because other services are running. If there was a "pre run" and "post run" command I could tag into, I could start and stop those services and prep the environment for snapraid to run.

[omarmir]

Would love this feature. I would like to integrate Pushover and possibly a dashboard I'm building for my various monitoring services etc.