Function: get_metadata

--Usage--
> get_metadata(input: image)

Returns the metadata of an image, or an empty string if there isn't any.

Metadata is only stored when an image is saved as a PNG.
It is saved in a uncompressed iTXt chunk with the key "Description".
If the image is saved as a JPG, the metadata is lost.

--Parameters--
! Parameter	Type				Description
| @input@	[[type:image]]		The image whose metadata we'll get.
| @set@		[[type:set]]		The active set. This can be omitted since 'set' is a predefined variable.