feat: custom script and more

[farfromrefug]

This PR adds bug fixes and new features:

• feature: you can now customize i18n from command line / config a04ddda
• feature: customScript option: if a path to a js file is passed then it will be loaded and can be used to have custom behavior. For now the only implemented example is fetchRssItemImage which is used for custom fetch of rss images. In my case i use it to get episodes images from the podcast website.
• feature: story audio duration added to story.json. This wont be used by lunii but can be used by "players" for easy and fast access to stories duration
• fix regression for when i added -generated to images generated with text. The regression was causing TTS to be computed on every run ...
• fix : stage names and tts is now based on a metadata.json file generated with items metadata. That way we dont rely on file name which can be "simplified" compared to the original item name. The benefit is better TTS and correct names in story.json for "players"
• fix: correctly query rss main image
• fix: if a podcat item has a "subtitle" use that for name. Usually better than item name (cleaner as item name often is Podcast title: episode title
• fix: skipImageConvert was ignored for top level image