Ignore array elements that do not have the key.
Allow merge_key to be an arbitrary JSON pointer
rename overwriteByKey, match_key.

overwriteByKey -> arrayMergeById
match_key -> idRef
'id' is now default value for 'refId'
Don't pass extra kwargs to descend.

This breaks forward compatibility with yet unknown mergeOptions.
Add ignoreId option.
Raise an error if 'items' is an array.
Test what happens with non-unique ids
Document arrayMergeById in README