implement "pip self-check" with automatic invocation by default

initial work cribbed from PR #1214 but then I changed direction a little
What's the point of this variable? Wouldn't it just be simpler to do:

```python
if not options.disable_self_check:
    from pip.commands import self_check
    with self._build_session(options) as session:
        self_check.maybe_check(session)
```

Or is there a goal that I'm not seeing?