Allow markdown-it as a peerDependency in version ^11.0.0 as well

[peschee]

Fixes #31 by allowing to have markdown-it in version ^10.0.0 or ^11.0.0.

[coveralls]

Pull Request Test Coverage Report for Build 59

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 97.938%

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Totals
Change from base Build 53:	0.03%
Covered Lines:	50
Relevant Lines:	51

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💛 - Coveralls

[nagaozen]

Using peerDependency with markdown-it has proven to be a bad idea. Though the project clearly states that it follows semver, major bumps are common without any API breaking changes. It's better for us to just remove the peerDependency from package.json

[peschee]

@nagaozen I have removed the markdown-it peer dependency entry.