Add initial output=md support

[strzibny]

This adds initial support for Markdown and fixes #10

[ilyazub]

Opus 5.5 Max wrote this:
#17 is merged (7acfb80) and covers this PR: output=md now prints the Markdown for search and archive, in any case, along with html, plain and csv, built on the same getRaw/SearchRaw/isRawOutput shape as here; it closed #10. Against main this PR now conflicts in 5 files (README.md, pkg/api/client.go, pkg/cmd/search.go, pkg/output/output.go, pkg/output/output_test.go). Unless something here is still missing from main, it can be closed.