Don't check the same URL multiple times

[chriselion]

Following up from a comment

If the same URL appears twice in a file, you can cache the results of the first check and not check it again.

Ideally if we could pass multiple files at once, this caching would persist across files.

[sanmai-NL]

The initial solution can be simple and without persistent caching: keep a dictionary/mapping URL -> status outside the file loop.

[tcort]

There's an open task for this link-check issue #31. Feel free to implement it and submit a PR. It'd be a nice feature to have.