Add value from environment variable #58

I want to add automatic generated values from build process into my MD files (i.e. versions determined while build) without the need to implement an own extension. Referencing those values i.e. as

<!-- AUTO-GENERATED-CONTENT:START (ENV:name=myFooVariable) -->
<!-- AUTO-GENERATED-CONTENT:END -->

would allow me to add variable like

myFooVariable=42 md-magic --path './**/*.md'