Show a prompt when interactive

[bilogic]

Related to this: #724

1. My use case is for the user to paste their contents into the CLI and press ctrl + d
2. My CLI command is age --passphrase --output /encrypted.age
3. So, after I press enter to auto generate a passphrase, it looks like this

   Enter passphrase (leave empty to autogenerate a secure one): 
   Using the autogenerated passphrase "patch-they-author-quiz-correct-silly-style-tape-fetch-egg".
   

4. A first time user will likely think something is still going on. Instead, a better output would be

   Enter passphrase (leave empty to autogenerate a secure one): 
   Using the autogenerated passphrase "patch-they-author-quiz-correct-silly-style-tape-fetch-egg".
   Paste/type your sensitive data and press ctrl + d to send
   
   

5. I don't want to store data as-is into a file as that would possibly leave a trace on the filesystem
6. A --prompt="Paste/type your sensitive data and press ctrl + d to send" would help