RSS-Bridge currently sanitizes the format name only for the display
action, which can cause problems if other actions depend on formats
as well.
It is therefore better to do sanitization in the factory class for
formats. Additionally, formats should not require a perfect match,
so 'Atom' and 'aToM' make no difference. This will also allow users
to define formats in their own style (i.e. only lowercase via CLI).
References #1001
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