Add a cucumber feature for variable includes
implement variable retrieval in include tag

Fixes #1464.
Fix code to allow dots in variables
extend feature to ensure it works with parameters as well
Document variables in include tag
Move catching of liquid errors in include tags

The previous code only caught render-time errors. This change
makes it catch parse-time errors as well, such as unknown tags.
output correct path in case of liquid error in layout

As with includes, an error in a layout was reported on the
page that used the layout. Fix this by passing on the path
of the layout that is rendered.
Merge pull request #1601 from maul-esel/fix-error-file

Fix error file reporting
Update history to reflect merge of #1601
Merge pull request #1495 from maul-esel/variable-include

Variable {% include %}
Update history to reflect merge of #1495