Implement 'batch' filter

[flexferrum]

Implement 'batch' filter according to the jinja2 specification:

https://jinja.palletsprojects.com/en/2.10.x/templates/#batch

A filter that batches items. It works pretty much like slice just the other way round. It returns a list of lists with the given number of items. If you provide a second parameter this is used to fill up missing items.

For instance, batch(3, 0) applied to the list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16] should give the list of three lists:

[[1, 2, 3, 4, 5, 6],
 [7, 8, 9, 10, 11, 0],
 [12, 13, 14, 15, 16, 0]]