allow strict parsing (to disallow key duplicates in mapping)

[niamster]

By default YAML parser ignores dupes, but it may be interesting to catch them and fail.

Example of failure for sample config

diff --git a/test/my-data.yaml b/test/my-data.yaml
index aa0f52f..2dcf546 100644
--- a/test/my-data.yaml
+++ b/test/my-data.yaml
@@ -1,4 +1,5 @@
 root:
   key1: value1
   key2: 1
+  key2: 1
 root2: test

Output:

panic: yaml: unmarshal errors:
  line 4: key "key2" already set in map

[niamster]

All required changes are already synced from upstream.