Idea: warn on unsupported keywords (e.g. "const" in draft-04 schemas)

I just spent a few minutes digging into a test failure which boiled down to the following:

from jsonschema import validate

schema = {
    "$schema": "http://json-schema.org/draft-04/schema#",
    "const": None,
}
validate("literally anything validates", schema)

which is of course because the const keyword was added in draft-06! Since I've seen analogous problems several times now, I wonder if we should make the earlier DraftXValidators emit a warning when they see a keyword which was added in a later draft. That would be:

• <= draft04, warn on propertyNames, contains, and const (added in draft06)
• <= draft06, warn on if, then, and else (added in draft07)
• In draft07, the media keyword was replaced with contentMediaType and contentEncoding, so warnings on both sides could help
• <= draft07... there are lots

I've ignored the string format values, because they explictly permit ignoring unknown values and it's reasonable to think that users might have backported a check - whereas the too-new keywords are almost certainly a programmer error.

Start with the validate example and the DraftXValidator implementations. Review how each draft handles schema keywords, then define warning behavior for the listed newer or renamed keywords while preserving the existing treatment of unknown format values; verify the warnings across the affected draft validators.