Allow link patterns to be passed via extras dict

[Crozzers]

This PR allows the user to pass in link patterns via the extras argument.
Previously, the user would have to remember to set two arguments correctly:

# correct
markdown(text, extras=['link-patterns'], link_patterns=[...])
# incorrect
markdown(text, link_patterns=[...])  # doesn't raise but doesn't activate extra
markdown(text, extras=['link-patterns'])  # raises error
markdown(text, extras={'link-patterns': [...]})  # raises error

This PR enables that last use case

[nicholasserra]

LGTM thanks for your continued support