Add hooks about eryn

[UnexomWid]

Pre-render hooks can actually be invoked at compile-time, because the content itself is a file and doesn't depend on the state (context and local).

Post-render hooks are a bit more complicated, because the content isn't file content anymore; it's rendered content. Therefore, it has to happen at render-time. This adds some complexity to the renderer, as it currently writes directly to the output buffer. If post-render hooks are present, the renderer has to pass the content to the hook via the bridge, and then it has to overwrite the previously-rendered content, while also readjusting the buffer size accordingly.

Also, hooks should correspond to a single eryn instance, in order to avoid unwanted hook invocation in other modules. Currently, there is no support for multiple eryn instances (as there is a single, global instance, which isn't ideal).

Since post-render hooks may have a non-negligible impact on the renderer performance, and since multiple eryn instances are not supported yet (but are planned to be), hooks will be introduced in a later release, such as 0.3 or 0.4.