Add option to markdown to open url in new window
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### Description
There's nothing more frustrating when reading a wiki entry and/or documentation and when you click on the link you're taken there - but in the same page you were just viewing. Obviously you can just hold down CTRL+mouse click to get the link opened in a new tab (or long press on mobile and choose Open in new tab) - but that's not really good workflow. I want to be able to write documentation which opens the link in a new tab if I want the user to stay on the page they're on.
Currently there is not a way within Gitlab Markdown to open a link in a new window. Sure you can use inline HTML:
```
<a href="http://example.com/" target="_blank">example</a>
```
but that's not markdown and inline HTML doesn't always work.
~"feature proposal"
### Proposal
I believe that Jekyll, pandadoc and a few others use the following syntax `[link](url){:target="_blank"}` which could be an option
### Links / references
https://gitlab.com/gitlab-org/gitlab-ce/blob/v8.12.0/doc/user/markdown.md
issue