The imag-view command was redesigned with this change. It now looks up
view-templates in the config and calls them with the "in" subcommand.
This way, an entry can be viewed with editor, browser or whatever one
likes.
Compiling markdown is not supported yet.
This patch removes unused crate imports reported by newer rust versions.
Some crates were only required for tests, some only for tests with
macro_import - these things were fixed with feature gates.
