Add optional to disable strip path and enable uppercasing the name

[IonutBajescu]

This commit adds two new cli flags:

1. -strip-path Will remove the AWS_ENV_PATH from the environment
   variable, useful when you need to call aws-env multiple times and want
   to ensure that the environment variables exposed won't conflict with
   each other.
2. -uppercase Changes the capitalisation of the variable's name to
   all-caps. It is common for ssm parameters to be in all-lowercase whereas
   we obviously want the environment variables to be in uppercase.

[IonutBajescu]

@orfin Any chance you could take a look at this PR? Cheers