Show list opts when inspecting Regex

[sabiwara]

Hi,

This is a proposal to improve inspection of Regex built from an atom list.

iex(1)> Regex.compile!("foo", [:caseless])
~r/foo/

This issue was mentioned in this forum .

Feel free to close if we want to keep it this way or follow a different approach 🙂

[josevalim]

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