Bypassing youtubedl crash when creating .cache dir

[pafontai]

When downloading video, youtube-dl uses by default a cache directory.
This directory is located at ~/.cache/youtube-dl by default

When running into a webserver in ubuntu, youtube-dl tries to create /var/www/.cache and crashes because this operation is forbidden.

I could bypass this error by adding parameters at "youtubedlExe" configuration into "config.php"
'youtubedlExe' => '/usr/local/bin/youtube-dl --cache-dir /var/www/html/youtube-dl-webui/tmp ',

I propose two improvements :

• adding a new parameters youtubedlCacheDir
• Tell the user into the guidelines that it's able to add parameters to control youtubedl executio