added default elixir formatter (#934)

ftplugin/elixir.lua

@@ -2,6 +2,21 @@ if require("lv-utils").check_lsp_client_active "elixirls" then
   return
 end
 
+O.formatters.filetype["elixir"] = {
+  function()
+    return {
+      exe = O.lang.elixir.formatter.exe,
+      args = O.lang.elixir.formatter.args,
+      stdin = not (O.lang.elixir.formatter.stdin ~= nil),
+    }
+  end,
+}
+
+require("formatter.config").set_defaults {
+  logging = false,
+  filetype = O.formatters.filetype,
+}
+
 require("lspconfig").elixirls.setup {
   cmd = { DATA_PATH .. "/lspinstall/elixir/elixir-ls/language_server.sh" },
 }

lua/default-config.lua

@@ -123,7 +123,13 @@ O = {
     efm = {},
     elm = {},
     emmet = { active = false },
-    elixir = {},
+    elixir = {
+      formatter = {
+        exe = "mix",
+        args = { "format" },
+        stdin = true,
+      },
+    },
     graphql = {},
     go = {
       formatter = {