Introduce in-place array merging

[OndraZizka]

Sometimes, the array items should be written into based on the index rather than added to the other array.
There should be an option for choosing between modes, like -DarrayMerge=(append|index|<mapKey>). Or perhaps the YAML could contain some metavalues, like:

foo:
- content: this will be overwritten
- content: this will stay
---
foo:
- yamlMerge.mode: index
  content: a new value for array[0]