Support the `.markdown` file extension for markdown files

[aloisklink]

:bookmark_tabs: Summary

Support using markdown files as an input/output that have a .markdown file extension, instead of just .md.

Resolves #471

:straight_ruler: Design Decisions

I've basically just added .markdown everywhere I could find .md.

I haven't bothered adding it to the mmdc --help text, since I feel like most people use .md, not .markdown, and I don't want to make the --help text more complicated.

:clipboard: Tasks

Make sure you

• [x] :book: have read the contribution guidelines
• [x] :computer: have added unit/e2e tests (if appropriate)
• [x] :bookmark: targeted master branch

[MindaugasLaganeckas]

You rock! 😄