A `--baseline-report` command (also a report for normal invokations)

From #117 and #185 and others

To provide an overview and insight into the current state of the baseline.

Here is a mockup of what information it could contain:
```
> mypy --baseline-report
test.py:1:1: error: sus type error [😳]
test.py:2:1: error: sus type error [dynamic]
test.py:3:1: error: sus type error [misc]
test.py:4:1: error: sus type error [misc]
asdf.py:1:1: error: sus type error [😳]
asdf.py:2:1: error: sus type error [dynamic]
asdf.py:3:1: error: sus type error [misc]
asdf.py:4:1: error: sus type error [misc]
8 errors in 2 files

File Summary:
test.py: 4 errors
asdf.py: 4 errors

Error Breakdown:
😳: 2
dynamic: 2
misc: 4

Currently Adopting:
disallow_any_expr
disallow_untyped_defs
```

The issue names no files or tests and refers to related issues #117 and #185. Start by reviewing those discussions and locating the existing baseline and report handling; the work is done when the required baseline summary, file summary, error breakdown, and adoption information are specified and covered by tests.

Written by the indexing model from the issue text.