include a preheat_whitelist config option

[havok2063]

Problem

For the cases when voila is serving a large directory of notebooks, but you only want one or two to be preheated, it would be nice to be able to whitelist those few notebooks, rather than blacklist all other notebooks. Otherwise this could produce a very long list of notebooks in the blacklist that one needs to (remember to) maintain.

Proposed Solution

If it's not possible already, add a new config option, preheat_whitelist, which is a list of notebooks to explicitly preheat. It could behave the same as preheat_blacklist.

[trungleduc]

Thanks @havok2063, indeed it's a good idea since we have both black and white lists for static files.