Add DRY_RUN option

[zerthimon]

This patch provides DRY_RUN option to test the settings before the run.
ex:

DRY_RUN=1

[mbruggmann]

Could you squash your commits? 👍 for the functionality

[zerthimon]

zerthimon@a3b2d2a
Hm, this commit is the squash, I thought.

[zerthimon]

@mbruggmann anything else I can do to make this PR merged ?

[emmetog]

@zerthimon the PR now has 4 commits in it, when you've squashed all the commits into one there will only be 1 single commit in this PR.

Here's a guide that might help you:
https://github.com/ginatrapani/todo.txt-android/wiki/Squash-All-Commits-Related-to-a-Single-Issue-into-a-Single-Commit

[zerthimon]

@emmetog @mbruggmann

PR is fixed
(the wrong step from my side was to merge after rebase, had to do forced commit)

[mattnworb]

@zerthimon thanks!

[DeviaVir]

@mattnworb @mbruggmann the docker build for this change failed, any chance you could kick it?
https://hub.docker.com/r/spotify/docker-gc/builds/bhxisqpuqaotobglstrba58/

[mattnworb]

@DeviaVir thanks for the heads up. I triggered https://hub.docker.com/r/spotify/docker-gc/builds/buw596blrgvwjvp4g72imvr/