Populate share-alike flag

[migurski]

Fifth task for openaddresses/openaddresses-ops#7: introduce a new boolean value for share-alike licenses like ODbL, and add a True/False/Null output share-alike field.

Explicit share-alike requirement:

{
    "license": {
        "share-alike": true
    }
}

Missing license:share-alike should be interpreted as True, so this is also equivalent:

{
    "license": { }
}

Completely omitted share-alike information should be interpreted as False, equivalent to:

{
    "license": {
        "share-alike": false
    }
}