Support multiple aliases for same credentials

During pssh add, provide a way to add multiple aliases for the same record (eg. define a delimiter, repeatedly loop asking if there are more aliases to add, etc.)

[chrisputnam9]

Support for adding more aliases while editing is in https://github.com/chrisputnam9/pssh/tree/feature-sprint-2.5

I decided against putting in the 'add' prompt for now, but maybe later.