feat: add option to use external css rewriter in template_cache_generator.

The provided rewriter will be used to transform every css read by the
TemplateCache, the original css is provided on stdin, and result is read
from the stdout of provided binary.
test: add test for template_cache_generator using cssjanus as css rewriter.

Puts cssjanus inside test/io, and creating additional
cssjanus-combined.py that is  cssjanus.py merged with csslex.py, to have
single file 'binary'.
Test runs only if python is present on the running system.  The test
uses css with simple rules that are getting traslated for RTL locales.