Support applying subschemas conditionally

[mfulton26]

Draft 7 of JSON Schema addes Keywords for Applying Subschemas Conditionally.

It would be amazing to add support for if/then/else to liform-react. e.g.:

schema:

{
    "properties": { "power": { "type": "integer" } },
    "if": { "properties": { "power": { "const": 9000 } } },
    "then": { "properties": { "disbelief": { "type": "boolean" } } },
    "else": { "properties": { "confidence": { "type": "string" } } }
}

data 1:

{ "power": 12 }

form 1:

<form>
    <label for="field-power">power</label>
    <input id="field-power" type="number" step="1"/>
    <label for="field-disbelief">disbelief</label>
    <input id="field-disbelief" type="checkbox"/>
</form>

data 2:

{ "power": 42000 }

form 1:

<form>
    <label for="field-power">power</label>
    <input id="field-power" type="number" step="1"/>
    <label for="field-confidence">confidence</label>
    <input id="field-confidence" type="text"/>
</form>

Basically, if the if schema is valid then the then schema is applied, otherwise the else schema is applied (if else is defined).