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

[Sympatron]

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.