Add Doctor to check docs & specs

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

.doctor.exs

@@ -0,0 +1,15 @@
+%Doctor.Config{
+  exception_moduledoc_required: true,
+  failed: false,
+  ignore_modules: [],
+  ignore_paths: [],
+  min_module_doc_coverage: 70,
+  min_module_spec_coverage: 50,
+  min_overall_doc_coverage: 90,
+  min_overall_spec_coverage: 30,
+  moduledoc_required: true,
+  raise: false,
+  reporter: Doctor.Reporters.Full,
+  struct_type_spec_required: true,
+  umbrella: false
+}