Improve --exclude regular expression examples

[nathany]

This is what the documentation has right now:

--exclude <EXCLUDE>
          Exclude URLs and mail addresses from checking (supports regex)

You can exclude links from getting checked by specifying regex patterns with --exclude (e.g. --exclude example\.(com|org)). If a file named .lycheeignore exists in the current working directory, its contents are excluded as well. The file allows you to list multiple regular expressions for exclusion (one pattern per line).

The URL I was attempting to exclude is an unusual case. It's from the generated HTML for a book that is discussing error handling for an invalid web address:

Display the error that occurs when <code>url.Parse</code> is used with an invalid web address, 
such as one containing a space: <code>https://a b.com/</code>

The URL appears several times in the file but never as part of an A HREF. The result is:

✗ [ERR] https://a/ | Failed: Network error: dns error: no record found for Query { name: Name("a."), query_type: AAAA, query_class: IN }

It took me a while to figure out how to ignore this. --exclude a would result in 567 Excluded and variations like ^a or ^a$ didn't ignore it (based on the example in #1280). From the documentation, it wasn't clear that the solution was to add the scheme.

The solution: ^https://a/$

Later I found this example in lychee.example.toml, which include the scheme in the regex:

# Exclude URLs and mail addresses from checking (supports regex).
exclude = ['^https://www\.linkedin\.com', '^https://web\.archive\.org/web/']

Perhaps an additional example or a better example in the README would help?

I don't mind opening a PR for that (not for my specific example, but something like that linkedin example seems good). Before I do, I just wanted to double check that this is working as intended?

[mre]

Yes, we can add an additional example. The regex matches on the full URL including the scheme in order to support cases like this. Perhaps we find a way to better describe this behavior. Feel free to open a PR with a suggestion. We could also think about adding more examples to the lychee documentation website, which is also open source.