secrets: rework how we store secrets

[ghost]

From the updated readme:

IPFS and cjdns private keys, SSL certificates, and cjdns peering credentials,
are tracked by Git in a secret repository, in encrypted form.
We need to decrypt them for usage, and encrypt them for committing changes.

# initialize and decrypt
$ git clone https://example.net/secrets.git secrets/
$ echo "the-key" > ../solarnet.key
$ ./secrets.sh -d

# make changes and encrypt
$ vim secrets_plaintext/secrets.yml
$ ./secrets.sh -e
$ cd secrets/
$ git add secrets.yml
$ git commit -m 'Add some password or so'

You can also pipe the key instead of writing it to a file:

$ echo "the-key" | ./secrets.sh -d

[jbenet]

pretty nice!

[jbenet]

LGTM