GitHub Linguist support for adblock rules

[scripthunter7]

GitHub has been supporting adblock syntax highlighting since September 5th. This PR will help you turn it on.

Further informations:

• Syntax highlighter repository: https://github.com/ameshkov/VscodeAdblockSyntax
• How Linguist works: https://github.com/github/linguist/blob/master/docs/how-linguist-works.md
• Linguist overrides: https://github.com/github/linguist/blob/master/docs/overrides.md
• Linguist commit: github-linguist/linguist@e78ef71