add 'allow_html_redirect' configuration option to avoid printing warning when redirecting .html URLs

[boegel]

I'm porting documentation that currently uses Sphinx + .rst to MkDocs + md, and I would like to ensure that redirects are in place for old URLs like https://docs.easybuild.io/en/latest/Configuration.html.

To achieve this, I'm using mkdocs-redirects, as follows in mkdocs.yml:

plugins:
  - redirects:
      redirect_maps:
        en/latest/Configuration.html: configuring_easybuild.md

That's working fine (see https://easybuilders.github.io/easybuild-docs/en/latest/Configuration.html which correctly redirects), but mkdocs-redirects is logging warnings when .html URLs are redirected:

WARNING  -  redirects plugin: 'en/latest/Configuration.html' is not a valid markdown file!

That's annoying, since I would like to use mkdocs build --strict in CI to test changes to our documentation, and that exits with a non-zero exit code as soon there as any warnings.

The changes being proposed here allow configuring mkdocs-redirects with allow_html_redirects to silence the warning:

plugins:
  - redirects:
      allow_html_redirect: True
      redirect_maps:
        en/latest/Configuration.html: configuring_easybuild.md

[boegel]

I'm happy to make the extra effort in this PR to update the README file with a new section that documents this feature, and to implement a test to verify that the feature works as expected, but I would like to get some feedback first on whether this feature would be accepted for inclusing in mkdocs-redirects before doing so.

[boegel]

@oprypin Thanks for the feedback, I've opened an issue for this at #51

[jrappen]

@oprypin what's the current status here?

[jrappen]

should be ... not a valid md or html ..., yes?