feat: add porcelain output

[piksel]

This PR adds the --porcelain flag to help with using watchtower in scripts. When set, it uses a built-in template (porcelain.v1.summary-no-log) and writes notifications to stdout.
The output is meant to be stable and easily processed by common scripting tools like awk etc.

Versioning of the templates is done by passing the version to --porcelain as an argument, with only v1 being supported for now. That way we can add newer versions of templates without breaking scripts.

The output of the v1 template looks like this:

container_name1 (container_image1:tag): Updated
container_name2 (container_image2:tag): Fresh Error: no credentials

(fresh here just means that it's the latest version we know of, but checking failed so it might not be)

or:

no containers matched filter

Fixes #1336

[codecov]

Codecov Report

Merging #1337 (5fa306a) into main (08831f7) will increase coverage by 0.51%.
The diff coverage is 80.64%.

@@            Coverage Diff             @@
##             main    #1337      +/-   ##
==========================================
+ Coverage   63.82%   64.33%   +0.51%     
==========================================
  Files          23       23              
  Lines        1534     1598      +64     
==========================================
+ Hits          979     1028      +49     
- Misses        465      473       +8     
- Partials       90       97       +7     

Impacted Files	Coverage Δ
pkg/notifications/email.go	90.69% <ø> (ø)
pkg/notifications/notifier.go	85.13% <75.00%> (+0.20%)	⬆️
pkg/notifications/shoutrrr.go	73.33% <80.00%> (-1.37%)	⬇️
internal/flags/flags.go	87.56% <80.95%> (-1.56%)	⬇️
pkg/notifications/slack.go	95.12% <100.00%> (+0.25%)	⬆️

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