add support for Liquid files (.liquid)

[coren-frankel]

Although line comments in Liquid require a wrapping liquid block in one way or another, I've included the line syntax # without consideration for the {% %} encapsulation due to the unlikelihood that # TODO will be used outside of a comment context.

Liquid input:

{% # for i in (1..3) -%}
  {{ i }}
{% # endfor %}
# Not a comment
{%
  ###############################
  # This is a comment
  # across multiple lines
  ###############################
%}

Output:

# Not a comment

I'd be happy to make any changes that are suggested.

[alstr]

Great stuff, thanks.