Check for SRI/CORS in CSS files

[fulldecent]

html-proofer currently checks for SRI/CORS attributes in Javascript links.

It should also do this for stylesheet links.

This is NOT a new command-line option. This is correcting the behavior of the current --check-sri option to match behavior as documented.

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We can duplicate work for scripts here: https://github.com/gjtorikian/html-proofer/pull/388/files

Here is where scripts are checked: https://github.com/ilyalyo/html-proofer/blob/0cc1e95fcb86e505ad27614299f184aade44e5a2/lib/html-proofer/check/scripts.rb

Here is where we want links to be checked: https://github.com/ilyalyo/html-proofer/blob/0cc1e95fcb86e505ad27614299f184aade44e5a2/lib/html-proofer/check/links.rb

[fulldecent]

Please release 3.6.1 with this update.