Fix the setting for generating docs with the latest Python-Markdown

https://python-markdown.github.io/change_log/release-2.5/#backwards-incompatible-changes

The previously documented method of appending the extension configuration as a string to the extension name will be deprecated in Python-Markdown version 2.6

Summary of changes

When I installed and executed mkdocs for cheking improved documents, mkdocs didn't work following like:

$ mkdocs build
ERROR   -  Config value: 'markdown_extensions'. Error: Failed loading extension
"markdown.extensions.toc(permalink=true)".

Aborted with 1 Configuration Errors!

The versions were:

mkdocs: 1.0.4
markdown: 3.0.1

$ pip3 show mkdocs
Name: mkdocs
Version: 1.0.4
Summary: Project documentation with Markdown.
Home-page: https://www.mkdocs.org
Author: Tom Christie
Author-email: tom@tomchristie.com
License: BSD
Location: /usr/local/lib/python3.7/site-packages
Requires: Jinja2, click, livereload, tornado, Markdown, PyYAML
Required-by: mkdocs-material

$ pip3 show markdown
Name: Markdown
Version: 3.0.1
Summary: Python implementation of Markdown.
Home-page: https://Python-Markdown.github.io/
Author: Waylan Limberg
Author-email: waylan.limberg@icloud.com
License: BSD License
Location: /usr/local/lib/python3.7/site-packages
Requires:
Required-by: pymdown-extensions, mkdocs

After the changes, mkdocs works fine:

$ mkdocs build
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /Users/niku/src/distillery/doc
WARNING -  A relative path to 'api-reference.html' is included in the 'nav' configuration, which is not found in the documentation files

Checklist

[x] New functions have typespecs, changed functions were updated
[x] Same for documentation, including moduledocs
[x] Tests were added or updated to cover changes
[x] Commits were squashed into a single coherent commit

Licensing/Copyright

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bitwalker / distillery