Support --isolatable flag on migration command to avoid issues in multi-machine environments

[lex0r]

Fixes https://github.com/wintercms/winter/issues/1225

[bennothommo]

@lex0r would you be willing to provide a PR to the docs explaining this usage? Namely, explaining that there needs to be an external cache set up (and cache likely cannot be database or file) and you need to provide the --isolated flag in order to use isolated migrations.