Add pecl yaml parser extension

[andypost]

Speed-up config import 15% https://www.drupal.org/node/2769555

[andypost]

Needs PR to alpine linux for combined package of https://pecl.php.net/package/yaml

• 2.0 for php7
• 1.3 for php56

PS: example alpinelinux/aports#355

[andypost]

This still needs alpine package

PS anavarre/drucker#28

[andypost]

Proper config options can be found in patch
https://www.drupal.org/node/2712647#comment-11786390