Encrypt and Decrypt commands

[fboucquez]

Fixes https://github.com/nemtech/symbol-bootstrap/issues/154

• A valid password is required by default. Bootstrap will ask for the password, or you can provide one with --password.
• Added --noPassword to bypass the password validation. Private keys in generated addresses.yml and preset.yml will be in plain text!
• Added encrypt and decrypt commands.

@ravitej-aluru @Wayonb, if this PR goes in, you will need to update your scripts adding --noPassword