[include_code] Hardcodes content as content path

[mirekdlugosz]

pelican/plugins/liquid_tags/include_code.py line 100 refers to content directory, assuming it is name of main directory where Pelican will look for content. It should use PATH variable from Pelican settings. However, tags don't have access to all Pelican settings.

A solution should be found, so tag works with path as specified in pelicanconf.py, and not only default Pelican content path.