Add format-ignore-exit-code configuration

[sirreal]

Add a new configuration option to ignore formatter exit codes.

Some formatters use non-zero exit codes when formatting was successful. See #183.

For example, phpcbf uses a non-zero exit code that cannot be configured at this time: https://github.com/squizlabs/PHP_CodeSniffer/issues/2898

Fixes #183.

https://github.com/mattn/efm-langserver/issues/271 can likely also be closed with if this change lands.