Add mdformat formatter (#47)

Author: Libadoxon

README.md

@@ -27,6 +27,7 @@
 - [ ] [ktlint](https://github.com/pinterest/ktlint)
 - [ ] [ktfmt](https://github.com/facebook/ktfmt)
 - [x] [latexindent](https://github.com/cmhughes/latexindent.pl)
+- [ ] [mdformat](https://github.com/executablebooks/mdformat)
 - [ ] [mixformat](https://github.com/elixir-lang/elixir/)
 - [ ] [nixfmt](https://github.com/serokell/nixfmt)
 - [ ] [ormolu](https://hackage.haskell.org/package/ormolu)

lua/guard-collection/formatter.lua

@@ -118,6 +118,12 @@ M.latexindent = {
   stdin = true,
 }
 
+M.mdformat = {
+  cmd = 'mdformat',
+  args = { '-' },
+  stdin = true,
+}
+
 M.mixformat = {
   cmd = 'mix',
   args = { 'format', '-', '--stdin-filename' },