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

[KotlinIsland]

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

[KotlinIsland]

relates to #95

[DetachHead]

why restrict this feature to the baseline file? it would be nice to have in the normal output as well

[KotlinIsland]

This is especially relevant now that baseline doesn't store line numbers

[Zeckie]

Should this be divided up, so they can be selected separately (eg. in projects where there are thousands of errors, spread across lots of modules in different packages)?

• the individual baselined errors
• the "File Summary" (listing individual modules / files, potentially with a depth limit for large projects)
• the "Error Breakdown"
• the "Currently Adopting"

There could be one switch to turn them all on, but would be useful to be able to turn each on / off individually.