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.

* 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).

Fixes #1247.

This fixes a bug where any subsequent highlighted block with codehilite
would result in the omission of the style setting, because it was popped
off the dict. It would then fall back to pygments_style 'default' after
the first block.

Fixes #1240

Fixes #1156.

If pygments is installed and the version doesn't match the expected version.
then any relevant tests will fail. To avoid failing tests due to different
output by pygments, those tests will be skipped. The pygments tox env
sets the `PYGMENTS_VERSION environment variable, so that env will always
run those tests against the expected version.

* Pygments specific tests now only run when the pygments version installed
  matches the expected version. That version is defined in an environment
  variable (PYGMENTS_VERSION) in the 'pygments' tox env (see #1030).
* When the Python lib tidylib is installed but the underlying c lib is not,
  the relevant tests are now skipped rather than fail. This matches the
  behavior when the Python lib is not installed. The tox envs are now useful
  on systems which don't have the c lib installed.

* All non-language classes should always be assigned to the pre tag.
* The language identifying class should never be included with the
general list of classes.

Fixes #1032