Add new config command.

[telackey]

Refactor, moving config init to simply init and add a new set of commands for handling the config file for the stack command itself.

stack config show # Print the config
stack config get <key> # Show the value of a particular setting
stack config set <key> <value> # Set the value of a setting

Supported settings so far are:

# Set the default deployment target (eg, compose, k8s, etc)
deploy-to: k8s

# The kube config file to use for k8s deployment
kube-config: /path/to/file

# Whether to use ssh for git
git-ssh: true

# Set the default image registry to use
image-registry: registry.digitalocean.com/bozemanpass

# The distribution url to use for updates
distribution-url: https://github.com/bozemanpass/stack/releases/latest/download/stack