Provide Comparable expressions for postgres types

[prefanatic]

Is your feature request related to a problem? Please describe.
As a developer, I would like to use Comparable expressions on postgres types.

Describe the solution you'd like
As you would with sqlite Comparable expressions, it would be really nice to extend this to postgres types. For example, being able to query between PgDateTime ranges:

final query = _database.dates.select()
      ..where((tbl) => tbl.date.isBetween(startDate, endDate));

[simolus3]

I agree, and since the relational operators should work for datetime types in Postgres, it's probably as simple as making PgDateTime implement Comparable so that the extension becomes available.

[simolus3]

Added in fe57822, the date and timestamp columns can be used to form comparisons with the next release.