add gpgcrypt plugin

[vkareh]

Add a plugin that opens and saves GPG-encrypted files (.gpg, .pgp). On open, the file is decrypted via the gpg CLI; on save, it is re-encrypted using the original method (symmetric or asymmetric with the same recipients). New files saved with a GPG extension default to symmetric encryption.

To test, enable the plugin, and make sure you have gpg and a working gpg-agent.

For symmetric encryption:

1. echo "hello world" | gpg --symmetric -o /tmp/test1.gpg
2. pluma /tmp/test1.gpg
3. edit file, save
4. gpg --decrypt /tmp/test1.gpg
5. verify decryption is symmetrical (i.e. password prompt)
6. verify decrypted text

For assymmetric encryption:

1. echo "hello world" | gpg --encrypt --default-recipient-self -o /tmp/test2.gpg
2. pluma /tmp/test2.gpg
3. edit file, save
4. gpg --decrypt /tmp/test2.gpg
5. verify decryption is asymmetrical (i.e. smart card or yubikey required)
6. verify decrypted text

"Save As" to GPG:

1. Open plaintext file or create new one
2. Save As /tmp/test3.gpg
3. (there should be a password prompt)
4. gpg --decrypt /tmp/test3.gpg
5. verify decrypted text

"Save As" away from GPG:

1. pluma /tmp/test3.gpg
2. Save As /tmp/test4.txt
3. verify file saved plaintext with no encryption