Update nano from 2.8.7 to 2.9.0

This is a bugfix and maintenance release.

Tested as working on Samsung Chromebook Plus (aarch64).

Add zlibpkg dependency to ag package

Update libslz from 1.1.0-1 to 1.1.0-2

Update libvorbis from 1.3.3 to 1.3.5

Update autoconf_archive from 2017-03-21 to 2017-09-28

Update nettle from 3.3 to 3.4

Add CONTRIBUTING.md

Add Learn more section

Fix egghead.io link

Highlight buttons, controls and badges

Bold the buttons, controls and badges

Correct step sequences

Update libogg from 1.3.1 to 1.3.3

Remove mirror url

Update icu4c from 58.2 to 60.1

Update nginx package

Add squashfs package

Update mutt from 1.8.3 to 1.9.1

Add poppler package

Add pdfgrep package

Add lzop package

Add lzo package

Add lz4 and xz compression support

Compress manpages

Swap LZ4_SUPPORT and XZ_SUPPORT lines

Add lzo compression support

add proxychains package

Update apriconv from 1.2.1-1 to 1.2.2

Add avocado package

Update ImageMagick to 7.0.7-11

Update chromebrew_scripts from 1.0.0 to 1.1.0