feat(core): support permalink: null in frontmatter to disable permalink

[Mister-Hope]

Now users can set permalink: null to disable global permalinkPattern affect on certain pages.

This is more natural then setting permalink or permalinkPattern to page path in frontmatter.

[coveralls]

Pull Request Test Coverage Report for Build 8770375500

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 41.64%

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Totals
Change from base Build 8716911468:	0.0%
Covered Lines:	680
Relevant Lines:	1678

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💛 - Coveralls