add abort methods

[davidism]

Add get_or_abort and one_or_abort methods (and async variants), which will return a single row or call flask.abort(404). These are similar to get_or_404 etc from Flask-SQLAlchemy. They take arguments to customize the query execution as well as the abort call.

I decided not to port first_or_404 because these methods seem intended for detail pages, where first would not catch that the result may not be unique. one_or_abort covers that case.

get will always return a model instance, but one works on any select statement, which returns a tuple result. I added a scalar argument to control whether to return a single item or the tuple result. It defaults to true since the common use case will be to query a model rather than a tuple of columns. It's possible to introspect the select statement to determine whether a single model is being queried, but I felt explicitly passing scalar=False was preferable.

For the first release of Flask-SQLAlchemy-Lite, I wanted to keep it as simple as possible, waiting to see what may be needed from Flask-SQLAlchemy. These helper methods handle a really common pattern in web applications, and go beyond just tweaking SQLAlchemy to actually integrate with Flask. So I felt it was appropriate to have these in this "light" extension.