Convert Table children to union type for ts 2.3 support

[cetra3]

Fixes #1143

Checklist

• Enable CircleCI for your fork
• Include tests
• Update documentation

Changes proposed in this pull request:

Allows Typescript 2.3 to be used which changed some of the behaviour of children with React: microsoft/TypeScript#13618

Reviewers should focus on:

Whether this breaks anything in Typescript 2.2

Screenshot

[palantirtech]

Thanks for your interest in palantir/blueprint, @cetra3! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

good catch! this looks like the issue I was hitting in #1108

[adidahiya]

I tried this out and it works with TS 2.2.2

[cetra3]

Ok awesome! Are there any other components that might be affected by this?

[adidahiya]

from a cursory search for React.ReactElement across the codebase... I came across this one:
IHotkeysTarget