Can't handle links to .md files without the extension

`mdbook` compiles links to other .md files just fine even if you omit the `.md` extension.
But `mdbook-linkcheck` fails with an error stating that it did not find the file. I assume `mdbook-linkcheck` only searches for the literal path, but maybe a config option should be added to allow enabling the same behavior.
It could also be the default to match what `mdbook` does.