Add cats.io and IO monad

[adelbertc]

This PR serves two purposes

1. Add an IO monad in a separate package
2. Get a discussion going around how Cats wants to approach IO/Task. It seems there is a mismatch in the nature of async on JVM and JS and it's tricky to get a Task with the same API on both JVM and JS. Furthermore, many folks will be using Task for FS2, and perhaps others the Task from Scalaz. There is also a relatively simple formulation of Task in the form of a thin wrapper around Eval in dogs, but that works on the JVM only. There's another that builds on Task using IO and MVars... I'm sure there are other approaches I don't know about.

I am proposing the possibility of Cats just having a simple IO and letting users of Cats decide where they want to get their Task from and how much cross-platform support they want.

[codecov-io]

Current coverage is 89.59%

Merging #907 into master will not affect coverage as of f0e397f

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Review entire Coverage Diff as of f0e397f

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[DavidGregory084]

I think the new module needs adding to the command aliases (buildJVM / validateJVM) below & scripts/travis_publish.sh