Skip link checking for URL with prefix in config

[TjeuKayim]

Adds a configuration option to exclude certain URLs to be link checked, in addition to skip_anchor_prefixes #812.

Use case: Links to internal web servers.

# Configure the link checker
[link_checker]
# Skip link checking for external URLs that start with these prefixes
skip_prefixes = [
    "http://[2001:db8::]/",
]

Prior art

mdBook linkcheck

# If necessary, you can exclude one or more web links from being checked with
# a list of regular expressions
exclude = [ "google\\.com" ]

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Code changes

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[Keats]

Sorry for the delay, looks good thanks!