Replace -p with ENV Variable

[ambsw-technology]

It seems to me that the following will result in the deletion of many keys:

• ssm-diff init -p "a/b/c"
• ssm-diff plan -p "a/b/c"
• ssm-diff apply

It would be easy to forget to append the -p flag. If this would result in massive deletions, it seems like a very dangerous design. I suggest moving the -p behavior into an ENV variable. This ensures that it must be deliberately altered, not merely forgotten.