Fix relative markdown links with anchors

[jonasfranz]

Fix #3784

This is not the most elegant solution but it works.

[codecov-io]

Codecov Report

Merging #4058 into master will increase coverage by 0.01%.
The diff coverage is 73.33%.

@@            Coverage Diff             @@
##           master    #4058      +/-   ##
==========================================
+ Coverage   19.96%   19.97%   +0.01%     
==========================================
  Files         153      153              
  Lines       30482    30491       +9     
==========================================
+ Hits         6086     6091       +5     
- Misses      23483    23486       +3     
- Partials      913      914       +1

Impacted Files	Coverage Δ
modules/util/util.go	29.26% <73.33%> (+7.39%)	⬆️

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[sapk]

Please use https://golang.org/pkg/net/url/#URL.ResolveReference .
As it is the recommended way to do it.

[jonasfranz]

@sapk If I use URL.ResolveReference, the other test cases fail.

[sapk]

@JonasFranzDEV Only relative one are failing https://play.golang.org/p/kk0OHdtTg0i

[sapk]

And this could easely be check by u.IsAbs()

[lunny]

@JonasFranzDEV please send back port to release/v1.4