feat(scully): support for markdown options like {:target="_blank"}

[SanderElias]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

allow additional attributes to be added to target anchors. It uses the same syntax as jekyll and kramdownL

[someLink](http://somewhere.com){:target="_blank" class="lnk ext"}

Does this PR introduce a breaking change?

• Yes
• No

Other information