Added new "requiredProperties" keyword to supersede the "required" keyword

[jgonzalezdr]

Added boolean values to "required" such that when set to true all the properties present in "properties" are required (related to #659).

[awwright]

Unfortunately this was previously defined as a boolean, but with different semantics, and this behavior is implemented in more than a few packages for reverse compatibility, e.g. tdegrunt/jsonschema.

I've once suggested having a "requiredProperties" keyword that functions the same as "properties" does, except that all the keywords there are also required. There was an argument against this too and I forget what it was.

[jgonzalezdr]

Maybe the solution could be creating a new "requiredProperties" keyword with the semantic that I just proposed above, which by the way would have a "naming convention" homogeneous with the rest of validation keywords for properties. Then the "required" keyword could be deprecated at some point. Anyway both could coexist during some time if needed.

[handrews]

@jgonzalezdr we have a good discussion going on in issue #659, so I'd like to close this until that discussion resolves. Otherwise, people will think this is the approved solution and review it as such.

If we decide on this direction, we will re-open this PR. But the requiredProperties solution also seems worth discussing. Let's resolve that before having an active PR attracting attention.