Option to override use_directory_urls for redirect sources

[andgineer]

Now mkdocs-redirect create both source and target redirect URLs based on mkdocs option use_directory_urls
But if we had a legacy site with file-style URLs and now we need redirects from that old legacy to new shiny site with fancy URL redirects we are out of luck :(

So I end with that hack:
https://github.com/andgineer/TRegExpr/blob/master/docs/fix_folder_redirects.py

To fix that for example we can have mkdocs-redict option use different use_directory_urls for the redirect sources.

Also we could specify that for each redirect.