[Julia] Add tags & key_features

[SaschaMann]

This is a mix of @cmcaine and I's comments in #2763, as well as the feature boxes on julialang.org:

• https://github.com/exercism/v3/issues/2763#issuecomment-724792070
• https://github.com/exercism/v3/issues/2763#issuecomment-725050854

[ErikSchierboom]

<= 100 characters is not a lot. Some punctuation and nicer phrasing had to be left out :)

Yeah, but we don't want to make the blocks very big :)

[SaschaMann]

We still have a few chars left to make all of them exactly 100 chars long, if you have any ideas... :D

[cmcaine]

Nice!