Issue title: Missing <credits> category
 Issue body: Thanks for your hard work!

Is it possible to add "**credits**" category?  like that we can get tags like "**actor**", "**director**" etc.


Thanks again!
 Comments: 
 Comment 0: @freearhey I could work on that, because I would also like to add other features such as the programme link. Do we have to edit only scripts/commands/programs/save.js file?
 Comment 1: @mcastellaneta [convertToXMLTV()](https://github.com/freearhey/epg-grabber/blob/16d02ce1fa0df77a48a9bbd3d68be054d6a6cdfa/src/utils.js#L152-L234) function in `epg-grabber`, which is used to generate the resulting XML, should also be updated. That seems to be all there is to do.
 Comment 2: @mcastellaneta TVM for pull your commit, can you add tag `<date />`?

Thanks!
 Comment 3: @MLBStream Do you mean the creation date of the programme?
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