Adds site.preprocess

[fserb]

This is a first shot at adding site.preprocess().

[oscarotero]

Thanks!
For performance, I think you should use the same code of the processors:

• It iterates first the preprocessors, if it's empty we don't need to iterate all pages.
• It uses the concurrent helper, so all processors are executed in parallel, which is faster.

[fserb]

Ahn, sorry for that. Done.

[oscarotero]

Thanks! 👍