Add doubleNonFinite for IEEE 754 floats

[velveteer]

Addresses #3

[ysangkok]

Thanks for quickly developing this PR.

I noticed that it matches on the actual bytestring inside the JSON AST. This means that if the string is encoded e.g. using escape codes, it wouldn't parse. Granted, this is a very academic issue, but I thought it was worth pointing out.

For example, using Aeson:

ghci> decode @Double "\"\\u002dinf\""
Just (-Infinity)

But using this PR:

ghci> decodeEither personDecoder "{\"lol\": \"\\u002dinf\"}"
Left (InternalException (HError {path = "/lol", errorMsg = "Failed to parse Scientific: Failed reading: takeWhile1", docLocation = "\"\\u002dinf\"}", docDebug = "json_iterator [ depth : 2, structural : '\"', offset : 8', error : No error ]"}))
ghci> decodeEither personDecoder "{\"lol\": \"-inf\"}"
Right (Person (-Infinity))

I don't really mind too much, I think the PR is fine as is. I don't think it is likely that people will re-encode their Aeson JSON such that it uses escape codes for the non-finite IEEE 754 values.

[velveteer]

Yeah that's the trick with get_raw_json_token, whereas get_string escapes the string but will generate an exception if the value isn't actually a string. I will see if I can address this without losing performance. Although I do agree not handling escaped strings for this particular decoder should not be a major problem.