Support generics for TypeScript

[SamVerschueren]

It would be nice if we could do something like this

interface User {
    id: string;
    name: string;
}

const UserTable = db.table<User>('User');

const result = UserTable.find().exec();
//=> result is now recognized as `User[]` so we can map over it etc.

Something we have to take into account is that when using limit(1) or findOne() it doesn't return an array but a single instance. Not sure if we can define that somehow.

[BenjaminPalko]

@SamVerschueren #89 I did just that :)