Portal context class

[jrafidi]

Checklist

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

Changes proposed in this pull request:

Add a context property called blueprintPortalClass that all Blueprint Portals respect. This is very beneficial if you are displaying dialogs within a shared component/larger plugin and want to scope your styles. You could go through to every instance of Popover/Dialog/Tooltip and add the class yourself, but you're likely to forget to do it the next time you add something.

Not sure how to update documentation on this one. Also open to suggestions on the context prop name.

Side note: writing the tests for this was confusing/tricky because the tests started with 3 Portals in wherever enzyme puts them. I would have expected to start with an empty space to test. Is there a chance something isn't cleaning up properly?

[palantirtech]

Thanks for your interest in palantir/blueprint, @jrafidi! 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]

Let's add something like this to portal.md at the bottom:

@### React context

`Portal` supports the following options on its [React context](https://facebook.github.io/react/docs/context.html).
To use them, supply a child context to a subtree that contains the Portals you want to customize.

@interface IPortalContext

[jrafidi]

Update docs

Preview: documentation
Coverage: core | datetime