Add functionality for modifying EncryptedFile

[mottljan]

Right now if you want to modify an existing encrypted file, you need to first decrypt it, modify it and encrypt it again. It would be great to at least create some new method that would make this process easier. E.g. it could take a lambda, that would get decrypted content as a parameter and it would require a modified data as a return value. Then it would encrypt new data again.