Add exclude_origins option to internal links checker

[denisdefreyne]

The internal links checker has an exclude configuration option, which matches hrefs. This is often not enough, as sometimes it is useful to ignore all links in a file. This PR adds a configuration option exclude_origins, which is used for exactly that purpose.

For example, the following excludes all files under /api from the internal links checker, perhaps because it’s auto-generated and is known to be broken:

checks:
  internal_links:
    exclude_origins: ["^/api/"]