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, even though it's generally a good idea, might be overkill for this? If we want to keep them, we should make --porcelain take the version as an argument, like --porcelain v1, that way we can add newer versions of templates without breaking scripts.

Fixes #1336

[codecov[bot]]

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%)	:arrow_up:
pkg/notifications/shoutrrr.go	73.33% <80.00%> (-1.37%)	:arrow_down:
internal/flags/flags.go	87.56% <80.95%> (-1.56%)	:arrow_down:
pkg/notifications/slack.go	95.12% <100.00%> (+0.25%)	:arrow_up:

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.