markdownlint-cli2: support standard input

Author: c-y-p-e

lua/lint/linters/markdownlint-cli2.lua

@@ -1,6 +1,8 @@
-local efm = "%f:%l:%c %m,%f:%l %m"
+local efm = "stdin:%l:%c %m,stdin:%l %m"
 return {
   cmd = "markdownlint-cli2",
+  stdin = true,
+  args = { "-" },
   ignore_exitcode = true,
   stream = "stderr",
   parser = require("lint.parser").from_errorformat(efm, {