fix(markdown3) KeyError: configs

[Kristinita]

1. Summary

I fix critical error for Markdown 3 and higher.

2. Testing environment

• Windows 10 Enterprise LTSB 64-bit EN,
• Python 3.7.1,
• Pelican 4.0.1,
• Python Markdown 3.0.1.

3. Steps to reproduce

I create simple Pelican site → I download and add mdx_cite plugin → pelican content.

4. Behavior before pull request

ERROR: Could not process Sublime-Text\SashaSublime.md
  | KeyError: 'configs'
ERROR: Could not process Gingerinas\Грейзи-Фа.md
  | KeyError: 'configs'
ERROR: Could not process Sasha-Black\Sasha-Black-description.md
  | KeyError: 'configs'
ERROR: Could not process Gingerinas\АльFAQ.md
  | KeyError: 'configs'
ERROR: Could not process IT-articles\typo-reporter.md
  | KeyError: 'configs'
CRITICAL: RuntimeError: generator raised StopIteration

5. Behavior after pull request

No errors.

Thanks.