feature: preventing XSS attacks in Markdown files

[walkingdevel]

Feature for preventing XSS attacks and running malicious JavaScript code in Markdown files. Yep, it is parsing HTML via regular expressions, but it is a temporary solution. In the future, I'll implement it with the built-in library, when the markdown library will support escaping HTML tags via DOM parsing. Maybe, I will do it.

P.S: I use the PCRE library because the built-in library couldn't parse my expressions.

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