feat: secure passwords (#193)

[andrinoff]

Closes #193

Changes:

1. Password storing:

Previously all passwords were kept in the form of:

{
  "accounts": [
    {
      "id": "unique-id-1",
      "name": "John Doe",
      "email": "drew@floatpane.com",
      "password": "this-isss-inse-cure",
      "service_provider": "gmail",
      "fetch_email": "john@gmail.com"
    },
    {
      "id": "unique-id-2",
      "name": "Work Email",
      "email": "john@floatpane.com",
      "password": "some-pass-word-easy",
      "service_provider": "custom",
      "fetch_email": "john@company.com",
      "imap_server": "imap.company.com",
      "imap_port": 993,
      "smtp_server": "smtp.company.com",
      "smtp_port": 587
    }
  ]
}

Where passwords were kept as plaintext.

Now, using the go-keyring library, we store the passwords in the OS keyring securely.

New config file:

{
  "accounts": [
    {
      "id": "54053bcf-79dd-4f0b-8e82-fff0810abff3",
      "name": "Drew Smirnoff",
      "email": "me@andrinoff.com",
      "service_provider": "icloud",
      "fetch_email": "me@andrinoff.com"
    },
    {
      "id": "7712fa1c-ce0b-43a9-af3a-2d69d605acf0",
      "name": "Drew Smirnoff",
      "email": "me@andrinoff.com",
      "service_provider": "icloud",
      "fetch_email": "drew@floatpane.com"
    }
  ]
}

2. Test change:

Added keyring.MockInit to initialize a mocking memory store for testing purposes

3. New dependency:

"github.com/zalando/go-keyring" and all the dependencies that come along.

4. Migration:

The new changes will NOT require users to adapt to the changes.

The LoadConfig function has been changed to detect old config with the plaintext password and move it to OS keyring, deleting the plaintext detected.

5. Size of the binary:

10.8MB --> 11.2MB

[andrinoff]

demos generate correctly, no issues found.