[buildifier] Add `--mode=off` to allow to run just looking for lints (and their fixes)

[thejcannon]

👋 Howdy!

Title says most of it, but Buildifier has an unfortunate CLI where I can't break up its operations into formatting/fixing

# I can get it to just complain about formatting
buildifier -mode=check -lint=off
buildifier -mode=diff -lint=off

# I can get it to just fix formatting
buildifier -mode=fix -lint=off

# I can get it to complain about formatting _and_ lints
buildifier -mode=check -lint=warn

# I can also get it to fix formatting and fixing
buildifier -mode=fix -lint=fix

# But I can't get it to just focus on lints
buildifier -mode=off -lint=fix
buildifier -mode=off -lint=warn