Fix for the Table of Contents (fix #2896)

[wszymanski]

The table of Contents will be created properly when paragraph is a link - boosted slugify method.

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

[Siemienik]

I confirm that works perfectly:muscle:

Example:

Header	Current output	Expected output
## [Test](/have/a/nice/day)	test-have-a-nice-day	test

For me it was required to makes Table of Contents ([[toc]]) works.

@wszymanski Thank you!

[ulivz]

Thanks for your work, could you help write a unit test for slugify function?