array merge strategies

[nhomble]

Could the cli accept some argument (or env var) to adjust the array merge strategy? I would expect an array to be entirely overridden and this is a pattern I see in springframework. I am sure other people will have opinions and this problem has already come up in #12

$ cat a.yml
a: [1, 2, 3]
$ cat b.yml
a: [4, 5, 6]
$ yaml-merge a.yml b.yml
a:
  - 1
  - 2
  - 3
  - 4
  - 5
  - 6

I would have expected

a: 
  - 4
  - 5
  - 6