Add WithSeverity

[michaelpj]

Fixes #13.

Couple of questions:

• I've given it a derived Show instance, rather than, say, something like [DEBUG] <msg>. I think it's probably better not to get opinionated about formatting and just have a boring Show.
• I don't know how you like to name record selectors. I think it's useful to have the selector for severity, as you can see in the example! But I'm happy to call it something else.

I took the liberty of adding a couple of deriving extensions to default-extensions since it looked like you had put others there. But happy to just put them in the file.

[michaelpj]

On reflection I should probably add mapSeverity :: (Severity -> Severity) -> WithSeverity msg -> WithSeverity msg in order to do things like this:

suppressErrors :: LogAction m (WithSeverity msg) -> LogAction m (WithSeverity msg)
suppressErrors = cmap (mapSeverity (\s -> if s == Error then Warning else s))

[chshersh]

@michaelpj The changes look great! 💯

Feel free to add the mapSeverity function as well 🙂
It looks useful 👍🏻

[michaelpj]

Added mapSeverity, so I think this is good to go!

[chshersh]

Looks good 👍🏻