Apply substitutions from all directives at once

[mondeja]

The problem is explained at #208 (comment). #211 implements a workaround for the most common case but it would be nice if we can apply substitutions for both directives at once, probably using placeholders like Python-Markdown does:

https://github.com/Python-Markdown/markdown/blob/ec8c305fb14eb081bb874c917d8b91d3c5122334/markdown/util.py#L66-L78