Add list command for listing unapplied migrations

[minamijoyo]

It would be useful if we have a command which lists unapplied migrations to integrate CI/CD easily.

bash-5.0# go run ../.. list --help
Usage: tfmigrate list

List migrations.

Options:
  --config           A path to tfmigrate config file
  --status           A filter for migration status
                     Valid values are as follows:
                       - all (default)
                       - unapplied

bash-5.0# go run ../.. list --status=all 2> /dev/null
mv_bar1.hcl
mv_bar2.hcl
mv_bar3.hcl
bash-5.0# go run ../.. list --status=unapplied 2> /dev/null
mv_bar2.hcl
mv_bar3.hcl