Replaced the --enforce-whitespace option with --ignore-whitespace

Restores original behavior of enforcing whitespace by default. The
behavior was broken in a backwards incompatible way starting with commit
5f6d4bd4. Ignoring whitespace is now
opt-in instead of opt-out.

Ignoring whitespace by default has lead to a number of confused users,
myself included. CI would seemingly pass but isort --apply would still
make changes. Some users have asked for the original behavior be
restored and the new feature be behind a CLI argument.

Refs #378
Fixes #423
Fixes #481