Support async replacements

[nylen]

Currently if you want to replace something using a call to an external API you need to use some hacks: https://github.com/ClassicPress/ClassicPress/pull/496/files#diff-2e5af0be9bc4d0bc2b8d274f4d9ccd59a904d796bf7de199f5e1e7c1e0aa2d42R7

Adding a way to have an async replace callback would make this better.

[outaTiME]

It is something that I have in my roadmap, but in order to support this plugin I first need to do it in the core (applause), when I have some free time I will see to implement it