Add zettel title to pyzet grep output

[tpwo]

It is possible by using some git grep flags:

$ git -C 'testing/zet/zettels' grep -Ii --heading --break --all-match -e '^#\s.*' --or -e 'hello'
20211016205158/README.md
# Zet test entry
Hello there!

20211016223643/README.md
# Another zet test entry
Hello everyone

^#\s.* symbolizes the zettel title. --or is used to provide git grep with multiple patterns, and --all-match has to be used, because otherwise all zettels would be matched (because every zettel contains a title).

The side effect of this approach is the fact that the first line is colored as matched (i.e. full red), but I don't see another way without messing with git grep output in Python, which I'd like to avoid.

Because of that, this should be rather an optional flag rather than a normal behavior.