Fix #63 - Paginate

[bhagany]

Provides a paginate task that sorts existing entries, partitions them by page-size, and renders an html file for each of the resulting partitions. The render function provided to the task will be called for each file to be generated a data structure in the same shape as the one provided to the collection task, a map with :entry, :entries and :meta keys.

The assortment task (first discussed in #109) comes along for the ride here, because paginate is a subset of assortment.

This will probably conflict with other pull requests that I'm submitting at the same time, but they'll be trivial for me to fix as they get merged.

[bhagany]

I should have mentioned - there is one backwards-incompatible change to collection. The groupby option has been removed, because this is covered more natrually by assortment. It can be added back in relatively easily if you prefer.

[podviaznikov]

Code looks good to me.
Only one ask would be to add use of assortment and paginate tasks into the example project https://github.com/hashobject/perun/tree/master/examples/blog

[bhagany]

Only one ask would be to add use of assortment and paginate tasks into the example project

Certainly. I should have some time later today to do this for each of the open PR's.

[podviaznikov]

ok, so 2 PRs merged and now there is a small conflict

[bhagany]

Okay, I fixed this conflict.