feat: add multiple accounts at once (fetch email)

[andrinoff]

What?

Adds multiple account creation by appending the "Email Address" field with commas.

After pressing enter on the last step, creates multiple accounts in the config.json.

Why multiple accounts separated?

Well, the reasons are pretty simple:

1. To configure each one however you want. e.g. you want to change a name in just one
2. To be able to delete 1, without deleting every other one
3. To be able to add different S/MIME certificates and PGP keys (FEAT: PGP signing/encrypting #386 coming soon)

Why?

Closes #427