Add flatten-function

[TobiasNx]

Flatten is not added as function.
It can be helpful if we have an array of keywords with some keywords connected and some tokenized.

in:

{
	"keywords": [
		"dog",
		"cat",
		"horse,bird,dragon"
	]
}

If we would split the third keyword we would (which we cannot at the moment) we would get an array in an array. Then flatten would be nice.

result (does not work yet because of the asterisk-problem with spli-field):

{
	"keywords": [
		"dog",
		"cat",
		["horse" , "bird" , "dragon"]
	]
}

should:

{
	"keywords": [
		"dog",
		"cat",
		"horse",
		"bird",
		"dragon"
	]
}

See test-scenario:
https://github.com/TobiasNx/fix-FunctionalReview-Testing/tree/master/data/testing/flattening

[blackwinter]

Related to #100.

[fsteeg]

Will revisit after metafacture/metafacture-core#601.