Create merge pipe

[flohw]

Hi,

I created a merge pipe which works similarly to the union pipe except it didn't prevent duplication. It only merge two arrays. If one of the values is null then it defaults to empty array.

Let me know if I need to add some tests or make some changes.

Thank you

[coveralls]

Coverage remained the same at 100.0% when pulling 91e48d45beeaec3bcf743b45753cdbfca11e4e5d on flohw:create_merge_pipe into 3bdddcfcd84afb7b5eb1c1f214e8ebb602a26c3d on fknop:master.