Add `rebar3_efmt` plugin

.gitignore

@@ -13,3 +13,4 @@ doc/*
 _build
 .*.sw?
 rebar3.crashdump
+.efmt/

rebar.config

@@ -55,3 +55,5 @@
 {project_plugins, [covertool]}.
 {cover_export_enabled, true}.
 {covertool, [{coverdata_files, ["ct.coverdata", "eunit.coverdata"]}]}.
+
+{plugins, [rebar3_efmt]}.