Yamllint: Allow to customize settings

Move default settings from template to .yamllint_defaults.yml and allow
customizations in .yamllint.yml.

.yamllint.yml

@@ -1,14 +1,17 @@
 # SPDX-License-Identifier: MIT
 ---
-ignore: |
-  /.tox/
-extends: default
-rules:
-  braces:
-    max-spaces-inside: 1
-    level: error
-  brackets:
-    max-spaces-inside: 1
-    level: error
-  truthy: disable
-  document-start: disable
+extends: .yamllint_defaults.yml
+# possible customizations over the base yamllint config
+# skip the yaml files in the /tests/ directory
+# NOTE: If you want to customize `ignore` you'll have to
+# copy in all of the config from .yamllint.yml, then
+# add your own - so if you want to just add /tests/ to
+# be ignored, you'll have to add the ignores from the base
+# ignore: |
+#   /tests/
+#   /.tox/
+# skip checking line length
+# NOTE: the above does not apply to `rules` - you do not
+# have to copy all of the rules from the base config
+# rules:
+#   line-length: disable

.yamllint_defaults.yml

@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: MIT
+---
+ignore: |
+  /.tox/
+extends: default
+rules:
+  braces:
+    max-spaces-inside: 1
+    level: error
+  brackets:
+    max-spaces-inside: 1
+    level: error
+  truthy: disable
+  document-start: disable