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.

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.

* 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.

This adds configuration support for using a custom Pygments formatter,
either by giving the string name, or a custom formatter class (or
callable).

Closes #1224

* footnotes: Allow to use backlink title without footnote number
- The placeholder '{}' is optional. So a user can choose to include or
not the footnote number in the backlink text.
- The modification is backward compatible with configurations using
the old '%d' placeholder.

* footnotes: Allow to use custom superscript text

- The addition of a new SUPERSCRIPT_TEXT option allows to specify
a placeholder receiving the footnote number for the superscript text.

This allows better interoperation with CSS style sheets, as the align
object on the TH is skipped if the css uses 'text-align: inherit' and
the previous 'text-align' is used instead (or the default: left).

Added an override to restore the original `align` behavior
Moved existing tests to the new test infrastructure
Added new tests to test the configuration parameter
Updated documentation to document the configuration parameter.

Fixes #1247.