Fix liquid_tags.img to be unicode-aware

As it was, liquid_tags.img would die when attempting to write a caption
for an image that contains non-ascii symbols.
Fix liquid_tags.include_code to be encoding aware.

Beyond the problem with parsing tags containing non-ascii symbols, this
plugin also makes the unfounded assumption that the code file does not
contain non-ascii symbols.  Unfortunately source code files are not
necessarily utf-8, unlike incoming post data...