Support json fields in the `fields` query option

[joelmukuthu]

Model.fields { foo: 'json' };
Model.table = 'data';
Model.fetch({
  fields: [{ foo: { bar: true } }, { foo: { quux: true } }]
});
// or with an alias:
Model.fetch({
  fields: { alias1: { foo: { bar: true } }, alias2: { foo: { quux: true } } } }
});

that yields:

-- without aliases:
SELECT "foo"->'bar' as "foo.bar", "foo"->'quux' as "foo.quux" FROM "data";
-- with an alias:
SELECT "foo"->'bar' as "alias1", "foo"->'quux' as "alias2" FROM "data";

however, since using an object allows one to use multiple keys, one could add multiple json paths for only one alias when using the aliased form which would result in undesired/unexpected behaviour. therefore, it makes sense to enforce a single key per object (in both the non-aliased and aliased forms for conformity).

note that the field value should get automatically JSON-parsed so it shouldn't be necessary to use the ->> operator:

postgres/lib/KnormPostgres.js

Lines 92 to 100 in d5c8b0c

	// only values coming from the database will be strings
	if (typeof value === 'string') {
	try {
	return JSON.parse(value);
	} catch (e) {
	// root-level string JSON values are already "parsed"
	// TODO: should this error be propagated?
	}
	}