Support LXC runtime

ISLET is now modular, to add LXC support do
1. Add `/opt/islet/modules/lxc` with at least `verfiy`, `attach`, and `start` functions (see `islet_shell`)
2. Add `/etc/islet/modules/lxc.conf` and add config variables for supported LXC options
3. Add to documentation and man page (`docs/islet.5`)
