Function: keyword_usage

--Usage--
> keyword_usage(card: some_card)

Returns a comma separated list of the keywords that appear on the card.

For example if a card contains the keywords "Flying" and "Haste" the function will return @"Flying, Haste"@.

If a keyword appears multiple times on the card it will also appear multiple times in the output, unless @unique@ is specified.

--Parameters--
! Parameter	Type				Description
| @card@	[[type:card]]			Card to find keywords on.
| @unique@	[[type:boolean]] (optional)	Return each keyword at most once.

--Examples--
> keyword_usage(card: some_card, unique: false) == "Flying, Flying, Haste"
> keyword_usage(card: some_card, unique: true)  == "Flying, Haste"
