This commit adds support for a new parameter which specifies the type
of cache to use for caching. It is specified in config.ini.php:
[cache]
type = "..."
Currently only one type of cache is supported (see /caches). All uses
of 'FileCache' were replaced by this configuration option.
Note: Caching currently depends on files and folders (due to FileCache).
Experience may vary depending on the selected cache type. For now always
check if FileCache is working before testing alternative types.
References #1000
Move CACHE_DIR from index.php to /lib/RssBridge.php and change name
to PATH_CACHE.
PATH_CACHE is one of the core paths of RSS-Bridge and should therefore
be defined in the core file RssBridge.php.
