This change adds an advisory mode via `--check` that only warns of
formatting issues with files, but does not address them.
This support is desirable because--while I don't mind the automagic
changes when done in a mechanical way--some individuals who I
described the current behavior of these fixers to were a bit uneasy
about the magic that went along with them. Adding `--check` so
others can opt out (similar to `black --check`) is a compromise on
this front.
Signed-off-by: Enji Cooper <yaneurabeya@gmail.com>