InvalidLinkBear: Support markdown files

[ghost]

Resolve matching for url of type [valid_url](valid_url)

Fixes #1338

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[sils]

ack 73fac5e

[sils]

@rultor merge

[sils]

@Mixih this is one of those who need backport

[rultor]

@rultor merge

@sils OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge

@sils Done! FYI, the full log is here (took me 1min)