Enable external link icon if link has a target _blank attribute

Problem/Motivation

Our contributors manage manually their external links using a target _blank attribute.

We need to be able to inject that external link icon based on the presence of that target _blank attribute.

Steps to reproduce

Currently the filter is only based on the content of the href attribute.

Proposed resolution

Add a configuration option allowing to enable the external link icon based on link href and/or target _blank attribute.