Support intermediate/subsidiary CAs

[Richard Terry]

I plan to implement this by adding an optional argument to init to specify the caman dir for the root CA, eg ./caman init /path/to/root_ca/.

Caman can then use the root CA to sign the intermediate CA and generate a chain file, at which point the intermediate CA would be a self-contained caman dir with its own config, and all other syntax would remain the same.

I have no pressing need for this, so I'll update this issue when I start working on it. In the meantime, comment here if this is something you're interested in, and of course patches would be most welcome.