feat: add support for pre-update and post-update messages

[sisp]

I've added support for pre-update and post-update messages that can be written in plain text. Messages can be written inline or included via Jinja includes, and messages are rendered.

This (simple) implementation became possible due to #1156 since there's no output of an internal copy call of the update algorithm anymore. Without this copy call output suppression, the internal copy call would print the pre-copy/post-copy messages in case they are configured which we wouldn't want to see when running an update.

Related to #723. Follow-up of #1194.

[codecov]

Codecov Report

Merging #1288 (42f9bef) into master (c5eb24f) will increase coverage by 0.11%.
Report is 5 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1288      +/-   ##
==========================================
+ Coverage   96.96%   97.07%   +0.11%     
==========================================
  Files          48       48              
  Lines        4053     4143      +90     
==========================================
+ Hits         3930     4022      +92     
+ Misses        123      121       -2     

Flag	Coverage Δ
unittests	97.07% <100.00%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
copier/main.py	96.17% <100.00%> (+0.01%)	⬆️
copier/template.py	96.44% <100.00%> (+0.09%)	⬆️
tests/test_output.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

[yajo]

Looks very nice. Just a suggestion for those regex