Motivation for this change

This creates a package for mdctags, a ctags generator for markdown files.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

How to test

test.md:

# Some big title

## Some other title

nix-build -A mdctags && ./result/bin/mdctags test.md

This extracts ctags:

!_...
Some big title	/home/kea/src/nixpkgs/test.md	/^# Some big title$/;"	a	line:1	
Some other title	/home/kea/src/nixpkgs/test.md	/^## Some other title$/;"	b	line:3	h1:Some big title

The program can be used with the tagbar Vim plugin: https://github.com/wsdjeg/mdctags.rs#config-mdctags-for-tagbar
I'm using it and it's working fine so far.