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>

Fixes #1274.

Cleaned up the release notes.

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.