In addition to checking the spelling in our documentation, we are now also checking the spelling of the README.md and similar files as well as comments in our Python code.

The `NOT_STRONG_RE` regex matchs 1, 2, or 3 * or _ which are surrounded by
white space to prevent them from being parsed as tokens. However, the
surrounding white space should not be consumed by the regex, which is why
lookhead and lookbehind assertions are used. As `^` cannot be matched in a
lookbehind assertion, it is left outside the assertion, but as it is zero
length, that should not matter.

Tests added and/or updated to cover various edge cases. Fixes #1300.

There are small typos in:
- docs/extensions/fenced_code_blocks.md
- docs/extensions/meta_data.md
- markdown/extensions/codehilite.py
- markdown/extensions/footnotes.py
- tests/test_legacy.py

Fixes:
- Should read `delimiters` rather than `deliminators`.
- Should read `shebang` rather than `sheband`.
- Should read `processing` rather than `proccesing`.
- Should read `backslash` rather than `blackslash`.

Signed-off-by: Tim Gates <tim.gates@iress.com>

By unescaping backslash escapes in a treeprocessor, the text is properly
escaped during serialization. Fixes #1131.

As it is recognized that various third-party extensions may be calling the
old class at `postprocessors.UnescapePostprocessor` the old class remains
in the codebase, but has been deprecated and will be removed in a future
release. The new class `treeprocessors.UnescapeTreeprocessor` should be
used instead.

This completely removes all objects which were deprecated in version 3.0 (this change will be included in version 3.4). Given the time that has passed, and the fact that older unmaintained extensions are not likely to support the new minimum Python version, this is little concern about breaking older extensions.

Fixes #1263.

* Add an extra option `lang_str` to pass the language of the code block
to the specified Pygments formatter.
* Include an example custom Pygments formatter in the documentation
that includes the language of the code in the output using the new option.

Resolves #1255.