Allows Typogrify to ignore user specified tags. Refs #1407

Typogrify interferes with certain sections of the output that it should not touch (see #1407 for more details).
This feature adds a setting called TYPOGRIFY_IGNORE_TAGS which is a list of tags for Typogrify to ignore.

The following was updated:

 1. readers.py - if TYPOGRIFY_IGNORE_TAGS is present, then use it
 2. contents/article_with_code_block.rst - an article with a code block for typogrify to ignore
 3. updated tests
 4. updated documentation