Fix a couple of issues

[ordinalM]

• onConfigLoaded() wasn't using the right parameter to check config, so updated the variable name
• there was an issue with the filename when caching the index page - if simply "/" has been used the $url parameter was empty, resulting in a dot file that was hidden and hard to clear. In the case of an empty $url I amended the cache filename to be "index".

[ohnonot]

So apparently this was never merged.
I'm creating a new pull request that incorporates your and some other ideas (see below), esp. an option to exclude some pages.

I'm not hopeful though, and I might create my own fork, although a few already exist: https://github.com/Nepose/pico_cache/network/members

I do hope to incorporate all the good stuff from the other forks, esp. incl. GET requests in the cached pages (e.g. separately cached pages for /index?page=2 and /index?page=3 etc.), and lastly tackle the PicoCMS wiki to replace this apparently abandoned project?