use passphrase on mac with bash script

[fawzi]

Using a passphrase is more secure, and on mac adding it to the keychain maintains the convenience when using the key. Fix the bash script to do that, the python script could be fixed analogously. If the service would accept an optional passphrase we could avoid storing a private key without passphrase at any point (always secure at rest).

[fawzi]

I think the convenience of doing the right thing and avoiding stupid mistakes (like piling up keys in the agent) still makes this worth to add (at least for mac users)