feat: add mdformat (nvim-lua#68)

* feat(formatters): add mdformat for markdown formatting

* docs: add mdformat to readme and conform.txt

README.md

@@ -193,6 +193,7 @@ To view configured and available formatters, as well as to see the log file, run
 - [jq](https://github.com/stedolan/jq) - Command-line JSON processor.
 - [latexindent](https://github.com/cmhughes/latexindent.pl) - A perl script for formatting LaTeX files that is generally included in major TeX distributions.
 - [markdownlint](https://github.com/DavidAnson/markdownlint) - A Node.js style checker and lint tool for Markdown/CommonMark files.
+- [mdformat](https://github.com/executablebooks/mdformat) - An opinionated Markdown formatter.
 - [nixfmt](https://github.com/serokell/nixfmt) - nixfmt is a formatter for Nix code, intended to apply a uniform style.
 - [nixpkgs_fmt](https://github.com/nix-community/nixpkgs-fmt) - nixpkgs-fmt is a Nix code formatter for nixpkgs.
 - [ocamlformat](https://github.com/ocaml-ppx/ocamlformat) - Auto-formatter for OCaml code.

doc/conform.txt

@@ -200,6 +200,7 @@ FORMATTERS                                                    *conform-formatter
               included in major TeX distributions.
 `markdownlint` - A Node.js style checker and lint tool for Markdown/CommonMark
                files.
+`mdformat` - An opinionated Markdown formatter.
 `nixfmt` - nixfmt is a formatter for Nix code, intended to apply a uniform
          style.
 `nixpkgs_fmt` - nixpkgs-fmt is a Nix code formatter for nixpkgs.

lua/conform/formatters/mdformat.lua

@@ -0,0 +1,9 @@
+---@type conform.FileFormatterConfig
+return {
+  meta = {
+    url = "https://github.com/executablebooks/mdformat",
+    description = "An opinionated Markdown formatter.",
+  },
+  command = "mdformat",
+  args = { "-" },
+}