add NikolaPygmentsHTML

This is a Nikola-specific Pygments HTML formatter, which produces code
identical to that of reST, but with `table` line numbers (if desired).
Code produced by the formatter is therefore compatible with existing
code.css files, and it also removes the need to write regexps to fix
things that didn’t comply.

The Markdown compiler still uses a regexp, another patch is needed for that.

Signed-off-by: Chris Warrick <kwpolska@gmail.com>
fix #1285 -- Add a modernized CodeBlock directive.

This is a saner and more modern rewrite of the CodeBlock directive
from reStructuredText.  This version defers the generation of all the
HTML to NikolaPygmentsHTML instead of a mix of a Pygments Lexer and a
custom hack.  As such, we can now use the `table` line numbers instead
of the broken `inline` mess that makes copy-pasting impossible.

Signed-off-by: Chris Warrick <kwpolska@gmail.com>
update changelog

Signed-off-by: Chris Warrick <kwpolska@gmail.com>
Merge pull request #1540 from getnikola/sanitize-codeblock

fix #1285 -- sanitize code blocks.