Expand (limited) environment variables in markdown parser

Description

Making dynamic README's is currently impossible, as they're always rendered from static markdown source into HTML.

Proposal

Make a markdown previewer that expands a limited set of environment variables. Files could have a custom extension to use that previewer and not the regular markdown one.

Links / references

Related SO question:
http://stackoverflow.com/questions/43743141/using-gitlab-variables-in-gitlab-readme-md-for-sonarqube-badges