Editable: Add prop types to warn on non-array value

[aduth]

Related: #2341

This pull request seeks to enhance Editable to log a warning when passed a non-Array value. As seen in #2341, it can be tempting to pass a plain text string to Editable, but these will not be parseable on subsequent editor sessions.

Testing instructions:

Verify that no warnings are logged for valid Editable usage (e.g. paragraph).

[Assuming #2341 has not yet been merged, ] verify that a warning is logged when rendering saved post with (non-formatted) Cover Image:

1. Navigate to Gutenberg > New Post
2. Insert a Cover Image
3. Assign an image to the block
4. Add some text to the cover image
5. Save post
6. Refresh page

[ellatrix]

I like this idea, but shouldn't we then have it for all props?

[aduth]

I like this idea, but shouldn't we then have it for all props?

I am conflicted, because as of yet I've mostly discouraged propTypes, since Facebook themselves do not use them internally and are already making strides in 16.x to move them externally, perhaps a sign of future unavailability.

With that in mind, in this case I was going to start with a constructor check on the prop, when realizing that propTypes is already so convenient (and also, this component has contextTypes, so it's hard to pretend they don't exist).

In retrospect, the constructor check may not have been all that ugly, if even the same result, and could help reinforce some consistency about how we (don't) use propTypes.

Do you have any thoughts?