Add map and sort_by Twig filters

[PhrozenByte]

This is intended as a additional last-minute feature for Pico 1.0.0. Feedback is highly appreciated!

Especially the sort_by filter is important to allow users custom sorting algorithms apart from renaming their content files. This also makes Step 4 of our Blogging section in the docs obsolete (it btw describes the problem a sort_by filter solves pretty well). Something like this is impossible to achieve with a Twig macro, so theme designers would otherwise very often need to also write a plugin. The map filter is just a by-product of the sort_by filter (better: of PicoTwigExtension::getKeyOfVar()).

What do you think?

[theshka]

I like the proposed changes @PhrozenByte, it only adds functionality and thats great. 👍 Perhaps including an example in the cookbook section of the docs and replacing the blogging instructions to use the new filter?