Include and extend file discover not relative to current file

[mrcljx]

Currently all includes and extends must be relative to the initial liquid-template (see https://github.com/sirlantis/grunt-liquid/blob/master/tasks/liquid.js#L44-L46).

So if you have index.liquid extending layouts/default.liquid and layouts/default.liquid includes head.liquid the folder structure must look like this (except the subfolder)

• index.liquid
• head.liquid
• layouts/
  • default.liquid
• subfolder/
  • index.liquid

If you render subfolder/index.liquid, index.liquid would have to include ../layouts/default.liquid which then would fail to include head.liquid (which would be expected at subfolder/head.liquid).