The subs attribute should be honored

[ggrossetie]

Here's an example:

:action: hello

[plantuml,subs=+attributes]
----
alice -> bob : {action}
----

In the above example, {action} should be replaced/substituted by hello.
You can use the following code to do that:

subs = attrs['subs']
content = parent.apply_subs(content, parent.resolve_subs(subs)) if subs