feat(cli): support reading message from stdin

[piksel]

Allows piping the message to shoutrrr CLI:

$ some_command | shoutrrr slack://xoxb:123456789012-1234567890123-4mt0t4l1YL3g1T5L4cK70k3N@C0123456789 -

When the SHOUTRRR_URL environment variable has been set, you can also just pipe directly to shoutrrr, as the default option is to read from stdin. The main reason not to do this for normal usage is to prevent confusion upon invoking the command without any arguments as it waits for input from the attached console.

$ export SHOUTRRR_URL="slack://xoxb:123456789012-1234567890123-4mt0t4l1YL3g1T5L4cK70k3N@C0123456789"
$ some_command | shoutrrr

[codecov]

Codecov Report

Merging #243 (e8f7c86) into main (6dbcb12) will increase coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   77.07%   77.55%   +0.47%     
==========================================
  Files          95       95              
  Lines        2844     2860      +16     
==========================================
+ Hits         2192     2218      +26     
+ Misses        473      469       -4     
+ Partials      179      173       -6     

Impacted Files	Coverage Δ
pkg/util/partition_message.go	90.00% <100.00%> (-5.46%)	⬇️
pkg/services/slack/slack_json.go	100.00% <0.00%> (ø)
pkg/services/discord/discord_json.go	92.59% <0.00%> (+0.92%)	⬆️
pkg/services/discord/discord_config.go	88.23% <0.00%> (+5.88%)	⬆️
pkg/services/discord/discord.go	81.25% <0.00%> (+24.72%)	⬆️

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