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

[Zac-HD]

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.

[PatMyron]

similar: #766